It’s a Tuesday morning, and you head outside to do a routine check on your AWTS unit — the kind of quick once-over you do every few months just to make sure everything looks right. You grab the lid, give it a lift, and stop dead. Staring back at you is a wasp nest the size of a football, built neatly into the underside of the access hatch. Dozens of European wasps are already in the air.
Or maybe it’s a different scenario: your alarm goes off in the middle of the night. You check the unit the next morning and find the blower has stopped — not because of a mechanical fault, but because a rat has chewed clean through the wiring.
These are not rare events. For Australian homeowners with AWTS units — particularly those on rural or semi-rural properties in NSW — pest incursions are one of the most common and least-discussed maintenance headaches. The combination of Australian climate conditions (heat, dry summers, proximity to bushland), and the unique characteristics of an AWTS unit (warmth, permanent moisture, organic food source, multiple access points), creates near-perfect conditions for a long list of pests.
This guide goes beyond the basics. If you want a shorter overview, we’ve previously covered Dealing with Bugs and Animals in an AWTS System on our blog. This post is the definitive resource — covering every major pest, the real damage they cause, and a practical, step-by-step guide to prevention and response, written specifically for Australian conditions.
European wasps are one of the most dangerous pests homeowners encounter in and around AWTS systems — they nest in enclosed, sheltered spaces and defend aggressively when disturbed.
Section 1: Why AWTS Units Attract Pests
To understand why your AWTS is a pest magnet, it helps to think about what the system actually offers: warmth, water, and food — three things every pest in Australia is constantly searching for.
Warmth. Your AWTS generates heat through biological activity. The bacteria inside the aeration chamber are constantly working to break down organic matter, and that process produces heat. In cooler months, the inside of an AWTS unit can be significantly warmer than the surrounding soil — which is exactly what insects and rodents are looking for when the temperature drops.
Moisture. Most AWTS units contain a permanent body of water in the aeration chamber. In a dry Australian summer, when creeks run low and water sources become scarce, your AWTS represents an accessible, reliable water supply. For insects, it’s also a breeding ground.
Food. Your AWTS is processing organic waste continuously. To a cockroach, a rat, or a blowfly, that’s not a deterrent — it’s an attraction.
What makes AWTS units especially vulnerable compared to older-style septic tanks is the number of access points. A simple anaerobic septic tank is largely sealed — one or two lids, not much else. An AWTS has air vents (for the blower), multiple inspection lids, blower pipes, irrigation outlets, and electrical conduit entries. Each of these is a potential entry point if not properly maintained and sealed.
The upshot: warmth plus moisture plus organic food plus multiple access points equals exactly the kind of environment Australian pests actively seek out.
Section 2: The Pests You’re Most Likely to Encounter
Cockroaches
Cockroaches are the single most common pest found inside AWTS systems in Australia. The two species most frequently associated with wastewater infrastructure are the Australian cockroach (Periplaneta australasiae) and the American cockroach (Periplaneta americana) — both large, reddish-brown species that thrive in warm, moist, organically rich environments.
They enter through poorly sealed lids, damaged gauze on vents, and gaps around blower pipe penetrations. Once inside, conditions are ideal for rapid breeding — dark, warm, and consistently moist with a permanent food supply. The real concern is what happens next: cockroaches found inside your AWTS are often the same ones turning up in your kitchen. They travel through drainage lines and can re-enter the home through floor drains, shower outlets, and sink traps.
Beyond the obvious revulsion, cockroaches carry pathogens including Salmonella and E. coli on their bodies and legs, and can contaminate surfaces wherever they travel.
Signs to watch for: Cockroaches visible around the unit during the day (they’re usually nocturnal, so daytime sightings suggest significant numbers), finding large cockroaches inside the home via drains, and a musty or unusual odour near the AWTS unit.
Cockroaches are the most common pest found in AWTS systems — they enter through vents and poorly sealed lids, and can make their way back through drainage pipes into the home.
European Wasps and Paper Wasps
Of all the pests that target AWTS units, European wasps (Vespula germanica) are arguably the most dangerous to deal with. They are not native to Australia but are now well-established across the southern states and ACT, and they have a particular fondness for enclosed, sheltered cavities — making the underside of an AWTS access lid or the inside of an access chamber close to ideal nesting territory.
European wasp nests grow throughout the warmer months. By late summer and early autumn in NSW — typically February through April — a colony that started in spring can be enormous. An undisturbed nest inside an AWTS lid can easily reach the size of a football or larger. Unlike European honeybees, European wasps are highly aggressive when their nest is disturbed, do not die after stinging, and can sting multiple times. Disturbing a large nest in a confined space is genuinely dangerous.
Paper wasps (Polistes species) are also common around AWTS units in Australia. They are less aggressive and build smaller, open-comb nests (you can see the individual cells) rather than the papery enclosed structure of a European wasp nest. They are worth treating carefully but are far less hazardous than European wasps.
Signs to watch for: Visible wasp activity around the unit — particularly wasps entering and exiting through a gap around the lid or vent. You may hear a low humming when standing near a closed lid. Wasps seen repeatedly flying to the same point on or near the unit are almost always returning to a nest.
Critical safety note: Before lifting any AWTS lid, do a visual check from a safe distance. If you see wasp activity, do not open the lid. Stand back and call a licensed pest controller.
Rats and Mice
Rodents — primarily the black rat (Rattus rattus) and the brown rat (Rattus norvegicus), along with the house mouse (Mus musculus) — are drawn to AWTS units for warmth, water, and shelter. In rural and semi-rural NSW, native rodents including bush rats (Rattus fuscipes) can also be responsible, particularly during drought conditions when natural food and water sources dry up.
Rat and mouse damage to AWTS systems is one of the most common call-outs for septic service technicians across Australia. The problem is almost always the same: rodents chew through electrical wiring. Your AWTS has a blower motor, a pump, a float switch, and an alarm — all connected by wiring that runs in conduit around and beneath the unit. Rodents, attracted by the warmth the conduit provides, will gnaw through the insulation and the wiring itself. The results range from a blown blower (which means no aeration, and your system will fail its treatment process in days) to a non-functioning alarm (which means you won’t know when something goes wrong).
Rodents will also chew through foam insulation, PVC pipe, and softer plastic components if motivated enough by access to water or food on the other side.
Signs to watch for: Chewed or frayed wiring (visible during a check of the unit’s electrical connections), gnaw marks on plastic conduit or pipe fittings, rodent droppings near the unit or around the base, and — very commonly — the AWTS alarm activating unexpectedly with no obvious system fault.
Mosquitoes
Your AWTS aeration chamber contains standing water. If the unit is not properly sealed, it becomes a mosquito breeding site — and in NSW and Queensland, that is not a trivial concern.
Several mosquito species present in NSW are vectors for Ross River virus and Barmah Forest virus, both of which cause significant illness including joint pain, fatigue, and fever that can persist for months. NSW Health AWTS operation guidelines and most local council approval conditions specifically require that all air vents on an AWTS unit be covered with fine gauze mesh to prevent mosquito access. This is not just a recommendation — it is a standard condition of approval in most NSW council areas.
A well-maintained, properly sealed AWTS should provide no mosquito breeding opportunity. If you’re noticing an increase in mosquito activity around your system, that is a clear sign that a seal has failed or that vent gauze has corroded or torn.
Signs to watch for: Increased mosquito activity near the unit — particularly at dusk and dawn. Larvae (small, wriggling creatures visible in standing water) near or accessible inside the unit. Damaged, corroded, or missing gauze on vents.
Ants
Ants are generally the least damaging pest on this list, but they are not harmless, and some species warrant serious attention.
Meat ants (Iridomyrmex purpureus) and bull ants (Myrmecia species) commonly nest in the warm soil around the base of an AWTS unit. Both are attracted to the warmth of the underlying infrastructure, and large colonies can undermine the soil stability around the unit if left unchecked. Inside the unit, ants can interfere with float switches and sensors — not by chewing them, but simply by building up around them in sufficient numbers.
The more serious concern in Queensland and parts of northern NSW is fire ants (Solenopsis invicta). Fire ants are an invasive species and a notifiable pest — they are under active eradication programs managed by Biosecurity Queensland and NSW DPI. Fire ants are particularly attracted to electrical equipment and are known to cause damage to wiring similar to rodents. If you suspect fire ants near your AWTS unit — identified by their reddish-brown colour, aggressive swarming behaviour when disturbed, and distinctive mound structure — do not attempt to treat them yourself. You are required by law to report them.
Signs to watch for: Ant trails leading to or from the unit, nesting mounds at the base of the unit, ant activity on or around electrical components, and (for fire ant suspicion) aggressive ants with a reddish colouration emerging from irregular mounds.
Possums
This one surprises homeowners, but it happens more often than you’d expect — particularly in summer.
Both brushtail possums (Trichosurus vulpecula) and ringtail possums (Pseudocheirus peregrinus) are common on rural and semi-rural properties across NSW. They are inquisitive, physically capable animals, and they are actively searching for water during dry summer periods. A loose or poorly fastened AWTS lid represents an investigation opportunity.
If a possum enters an AWTS tank, the outcome is almost always fatal for the possum. A deceased possum inside an AWTS unit is a serious contamination event. The decomposition process can overwhelm the biological treatment process, and the unit will typically need to be pumped out, cleaned, and reseeded with bacteria — a significant and avoidable expense.
There is also an important legal dimension. Possums are protected animals under NSW law (the Biodiversity Conservation Act 2016). You cannot trap, relocate, or harm a possum without a licence. This means that if possums are regularly approaching your unit, your options are limited to making the unit inaccessible — secured lids, covered vents, cleared vegetation — rather than removing the animal.
Signs to watch for: Scratch marks on AWTS lids or access hatches, lids that have been displaced or shifted, and unusual animal activity around the unit at night (possums are nocturnal).
Possums are protected under NSW law and cannot be trapped without a licence — but they can and do investigate AWTS lids and vents, particularly in dry summer months when they are looking for water.
Flies
Blowflies and house flies will breed in any organically rich, exposed environment. A properly functioning, well-sealed AWTS should offer no such opportunity — the unit is enclosed, and a healthy treatment process reduces odour dramatically. When flies are breeding around or inside an AWTS, it is almost always a sign of two problems occurring simultaneously: a compromised seal (providing access) and a treatment issue (producing the conditions flies need to breed).
Fly activity around your AWTS is not just unpleasant — it’s a useful early warning. If you’re seeing blowflies hovering around the unit or finding fly larvae (maggots) near access points, the unit needs professional attention.
Signs to watch for: Unusual fly activity near or around the unit, particularly blowflies. Larvae visible near inspection points or at the base of vents.
Section 3: The Damage They Can Cause
It’s worth being direct about what pest incursions actually cost — not just in inconvenience, but in repair bills and system downtime.
Chewed wiring is the most common and most expensive outcome of rodent activity. Replacing damaged wiring, blower motors, pumps, and float switches quickly adds up. Beyond the parts cost, a failed blower means your system stops aerating — and without aeration, your AWTS will fail its treatment process within days, potentially discharging inadequately treated effluent to your irrigation area.
Blocked vents caused by wasp or bird nesting reduce the airflow your blower needs to function correctly. Restricted aeration equals reduced treatment performance. You may not notice immediately, but over weeks your system’s effluent quality will decline.
Lid and seal damage from possum activity, weather, or general deterioration creates an open entry point for every other pest on this list. It also creates a contamination risk — an open AWTS is a public health issue if children or animals can access it.
Sensor and float switch interference from ants or debris can trigger false alarms or, worse, cause pumps to run dry or fail to activate when needed.
Animal drowning — most commonly possums — triggers an immediate biological crisis inside the unit. The decomposition of a carcass can rapidly overwhelm the bacterial population, crash the treatment process, and leave you with a unit that needs to be professionally pumped, cleaned, and restarted. The cost of that call-out, plus the period of system downtime, is entirely avoidable.
Mosquito breeding inside or adjacent to a poorly sealed unit creates a public health risk beyond your property boundary — particularly in areas where Ross River virus and Barmah Forest virus are circulating.
Section 4: Prevention — Your First Line of Defence
The good news is that most pest incursions are preventable. The following measures are practical, low-cost, and highly effective when applied consistently.
Inspect lids and seals regularly. At minimum, do a visual inspection of your AWTS unit every three months. Check concrete lids for cracks, plastic access hatches for warping or brittleness, and lid bolts for corrosion or looseness. Any gap or crack is a potential entry point. After significant weather events — particularly storms, high winds, or extended heat — check sooner.
Check and maintain vent gauze. All air vents on your AWTS must be covered with fine gauze mesh. This is a condition of approval under most NSW council AWTS operation certificates, consistent with NSW Health guidelines for wastewater management. Gauze corrodes, tears, and collapses over time — especially in coastal environments. Check it every service cycle and replace it immediately if it’s compromised. Standard insect mesh is insufficient; use gauze with an aperture fine enough to exclude mosquitoes (1mm or finer).
Keep the area clear. Mow the grass around your AWTS unit regularly and keep it trimmed back from the unit. Remove leaf litter, ground debris, and any timber or building materials stored nearby — these all provide shelter and harbourage for rodents, cockroaches, and ants. Overhanging branches provide possum access; trim these back.
Secure lids properly. Every access lid should be firmly bolted or fastened at all times — not just when you remember to. A possum, a large dog, or a curious child should not be able to shift the lid. If your lid fasteners are corroded or missing, have them replaced during your next service.
Check wiring during every inspection. Look at the wiring conduit running from the unit to the electrical supply. Rodent gnaw marks on conduit are an early warning that wiring damage may already have occurred. Don’t wait for the blower to fail.
Use pest-resistant materials when replacing components. If wiring conduit or electrical connections near the unit need replacement, ask your service provider about rodent-resistant conduit options — metal conduit or heavy-wall PVC is significantly more resistant than standard flexible conduit.
Remove food sources from the surrounding area. Avoid locating compost bins directly adjacent to your AWTS unit. Keep pet food bowls inside or away from the unit area. Clean up fruit fall from nearby trees. The less organic food material in the immediate vicinity, the less attractive the area is to rodents and insects.
Section 5: What to Do If Pests Are Already Inside
If you’ve already found evidence of pest activity in or around your AWTS, here’s how to respond — pest by pest.
Wasps
Do not attempt to remove a European wasp nest from your AWTS yourself. The enclosed space, large colony size, and aggressive defensive behaviour make this genuinely dangerous. Call a licensed pest controller. They will treat the nest (typically with dust insecticide applied into the entry point) and return when the colony is dead to remove it safely.
Paper wasp nests — identifiable by their small size and open-comb structure — are lower risk. Small, inactive paper wasp nests can sometimes be removed carefully by a competent homeowner wearing protective clothing. If there is any doubt about the species, or if the nest is large or actively occupied, call a professional.
Rats and Mice
Set snap traps or bait stations in a locked rodent bait station placed near (but not inside) the unit. Check and reset traps regularly — at least every 48 hours. Once you have confirmed rodent activity near your AWTS, contact both a licensed electrician and your AWTS service provider. The wiring must be inspected and tested before the unit is restarted — running a blower or pump on damaged wiring risks electrical fault and fire.
Do not use rodenticide bait inside or immediately around the AWTS unit itself.
Cockroaches
Treat entry points with boric acid powder or gel cockroach bait placed near (not inside) the access points, vents, and pipe penetrations. Boric acid and gel baits are low-toxicity to non-target species and will not affect the bacterial population in your AWTS if applied carefully outside the unit.
Do not spray any aerosol or liquid insecticide into or near the AWTS unit. Even small quantities of residual insecticide entering the treatment chamber can kill the beneficial bacteria your system depends on, causing treatment failure. If cockroach numbers are significant or you are finding them regularly inside the home via drains, contact your AWTS service provider — they can assess whether there is an internal infestation and advise on appropriate treatment.
Possums
Contact your local council or a licensed wildlife handler. Under the Biodiversity Conservation Act 2016, you cannot trap or relocate a possum in NSW without a permit — doing so is an offence. Councils can advise on legal deterrent options, and licensed wildlife handlers can relocate animals where necessary.
If a possum has entered the tank, call Eco Septic immediately on 1800 808 135. The unit will need to be taken offline, inspected, pumped if necessary, cleaned, and potentially reseeded with bacteria before it can be returned to service. The longer you leave it, the greater the biological damage to the system.
Mosquitoes
Check all vents and replace gauze immediately if it is torn, corroded, or missing. If larvae are visible in accessible water near or inside the unit, contact your service provider — this indicates a seal failure or a system issue that requires professional attention.
Ants
Diatomaceous earth applied around the base of the unit is an effective, non-toxic deterrent for most common ant species. Commercial ant bait stations (not sprays) placed near but not inside the unit can reduce foraging activity significantly.
For suspected fire ants, do not attempt any DIY treatment. Fire ants are a notifiable biosecurity pest in Queensland and NSW. Report sightings to Biosecurity Queensland (if in QLD) or NSW DPI. Attempting DIY treatment can spread the colony and is counterproductive to eradication programs.
Flies
Persistent fly activity around your AWTS is a symptom, not the root problem. Check and replace any damaged lid seals or vent gauze, then contact your AWTS service provider to assess treatment performance. A system that is generating sufficient odour and exposed organic material to attract blowflies is a system that needs professional attention.
Section 6: How Your Annual Service Protects You
A routine AWTS service call is about far more than just checking effluent quality. A qualified AWTS service technician will systematically inspect your unit’s seals, vent gauze, access lid condition, electrical connections, wiring, blower performance, pump function, and float switches — all the components that pests either target or that protect the system from pest entry.
A pest problem caught at a routine service is a minor issue. A wasp nest found and removed before it has damaged the lid gasket costs you a pest control call-out and a few hours. A rat nest found before it has severed the blower wiring costs you nothing but the inspection. The same problems left to develop between services can result in treatment failure, complete wiring replacement, unit clean-out, and council compliance notices.
Under most NSW council conditions, an AWTS must be serviced by a qualified service provider at regular intervals — typically every 3 to 12 months depending on council and system type. That scheduled service is your best insurance against pest-related damage becoming pest-related disaster.
If your service is overdue, your system is showing any of the signs described in this guide, or you have discovered evidence of pest activity in or around your AWTS, contact Eco Septic on 1800 808 135. We service AWTS units across NSW and can assess, repair, and advise on pest-proofing your system.
A well-maintained Eco Septic AWTS with intact seals, gauze-covered vents, and regular servicing is your best defence against pest incursion and system damage.
Ready to Protect Your AWTS?
Pests are a real and underappreciated risk to AWTS systems across Australia. The combination of warmth, moisture, organic material, and multiple access points makes these units attractive to cockroaches, wasps, rodents, mosquitoes, possums, and more — and the damage they cause ranges from nuisance to expensive to genuinely hazardous.
The good news: the vast majority of pest incursions are preventable with regular maintenance, intact seals, properly gauze-covered vents, and a service contract that keeps a qualified eye on your system throughout the year.
If you have concerns about your AWTS, have found evidence of pest activity, or simply want to make sure your system is properly protected, the team at Eco Septic is here to help. Call us on 1800 808 135 or get in touch through our website to book a service, repair, or consultation.
