Installing a septic tank isn’t as simple as digging a hole and dropping in a tank. It’s a carefully engineered process that requires compliance with local regulations, proper site assessment, quality workmanship, and the right materials. Whether you’re installing a concrete or poly (polyethylene) tank, doing it incorrectly can lead to severe financial, environmental, and legal consequences.
In this guide, we’ll show you the biggest installation mistakes people make when installing septic tanks—and how to avoid them. If you’re a DIYer, builder, or even a licensed plumber cutting corners, read this before you dig.
1. Skipping Council Approvals and Permits
What goes wrong:
One of the most common—and costly—errors is bypassing the required local council approval process. Whether it’s a concrete or poly septic tank, installation without proper approval is illegal and can lead to fines, forced removal, or denial of occupancy certificates.
Why it matters:
Septic systems involve public health and environmental protection. Councils and health departments require site-specific design assessments and will often request percolation or soil tests before giving the green light.
How to avoid:
Engage a wastewater consultant or plumber early.
Submit your wastewater design with your development application.
Wait for written approval before beginning any work.
2. Choosing the Wrong Location
What goes wrong:
Installing the tank too close to buildings, driveways, watercourses, or trees is a recipe for disaster. Tree roots can damage tanks and pipes, while proximity to water sources can cause contamination.
Why it matters:
Septic tanks must be located far enough away from sensitive areas and within easy access for maintenance trucks. Poor placement can cause flooding, contamination, and expensive excavation to relocate it later.
Common violations:
Too close to the house slab
Near a dam or well
Below stormwater lines or driveways
On unstable, flood-prone soil
How to avoid:
Stick to minimum setback distances outlined by your local council (e.g., 3m from buildings, 100m from bores)
Get a proper site and soil evaluation
Plan for vehicle access for pump-outs and servicing
3. Not Leveling the Base Properly
What goes wrong:
Failing to level the excavation base leads to an uneven tank that may tilt, settle, or even collapse. Especially with poly tanks, which are lighter, this is a common installation failure.
Why it matters:
An unlevel tank causes uneven stress, potentially damaging the baffles, pipework, and outlet systems. Over time, it may lead to leakage or structural failure.
How to avoid:
Use a laser level to ensure even base preparation
Compact the base with crushed rock or sand
Avoid backfilling unevenly, especially on one side only
4. Ignoring Buoyancy Risks
What goes wrong:
Installing a septic tank in a high water table or flood-prone area without proper anti-floatation measures can cause the tank to float—yes, float—even when buried.
Why it matters:
Poly tanks, due to their lighter weight, are especially prone to lifting out of the ground if not anchored or backfilled properly. But even concrete tanks can “float” if not full and installed in saturated soil.
Like a pool:
An empty swimming pool will float or crack in a high water table if not secured. Same goes for tanks.
How to avoid:
Fill the tank with water immediately after installation
Use concrete anchors or anti-floatation slabs in high water table areas
Avoid installing during rainy seasons unless managed carefully
5. Using the Wrong Backfill Material
What goes wrong:
Backfilling with large rocks, clay, or unfiltered fill can puncture a poly tank or place excessive pressure on a concrete tank, leading to cracks and leaks.
Why it matters:
Backfill must allow for even distribution and drainage. Poor backfilling causes deformation of poly tanks or stress fractures in concrete.
How to avoid:
Use crushed blue metal
Compact gently in layers
Never use machinery directly against the tank body
6. Poor Inlet and Outlet Connections
What goes wrong:
Using the wrong slope, incorrect fittings, or misaligned pipes can lead to constant blockages or sewage flowing in the wrong direction.
Why it matters:
Inlet and outlet pipes must be at the correct fall and securely sealed to prevent leaks, smells, or groundwater infiltration.
How to avoid:
Follow manufacturer specifications on pipe height and fall (usually 1-2% slope)
Always use rubber seals or grommets—not silicone alone
Check pipe alignment before backfilling
7. Neglecting Ventilation and Airflow
What goes wrong:
Installing without proper ventilation causes gas build-up inside the tank, leading to odours and potential pressure issues in the system.
Why it matters:
Hydrogen sulfide and methane gases are not only smelly—they’re dangerous. Adequate venting helps dissipate these gases safely.
How to avoid:
Ensure vent pipes are included and rise above roofline or garden beds
Use charcoal filters on vent outlets if required
Never cap or seal vent lines
8. Failing to Test Before Backfilling
What goes wrong:
Installing the tank, connecting it all, and covering it up—without any testing—leaves you blind to early leaks or installation faults.
Why it matters:
Once the system is backfilled and compacted, accessing it again means digging and more money. Testing early ensures everything works before you cover it.
How to avoid:
Fill tank with water and check for leaks at joints
Power up the control box and pump, ensuring everything runs
Perform an irrigation line flush test
9. Not Protecting the Tank During Construction
What goes wrong:
Leaving the tank exposed while other trades operate heavy machinery nearby can lead to cracks, scratches, or worst—damage that voids your warranty.
Why it matters:
Heavy machinery can collapse unfilled tanks. And exposure to sunlight over time (for poly tanks) degrades the plastic.
How to avoid:
Protect with temporary barriers
Backfill promptly once installation is tested
Ensure no vehicles drive over the tank or drainage field
10. Forgetting Ongoing Maintenance Access
What goes wrong:
Planting trees on top of the tank, concreting over the lid, or burying the access ports too deep is a short-sighted but surprisingly common mistake.
Why it matters:
You will need access for quarterly servicing, desludging, or inspections. Blocking access means more costly digging in the future—or worse, a failed system due to neglect.
How to avoid:
Leave access lids above ground or just under removable pavers
Clearly mark tank and irrigation locations
Keep garden beds, trees, and driveways far away from access points
11. Misunderstanding Poly vs Concrete Installation Needs
What goes wrong:
Assuming poly tanks can be installed the same way as concrete tanks is a recipe for problems. Each material type requires different handling.
Key differences:
| Factor | Poly Tank | Concrete Tank |
|---|---|---|
| Weight | Lightweight, needs anchoring | Heavy, resists buoyancy naturally |
| Backfill | Must be soft and compacted evenly | Can handle heavier backfill |
| Transport/Delivery | Easier with small machinery | Needs crane or HIAB |
| Site Prep | Must be perfectly flat and stable | More forgiving, but still critical |
| Long-Term Durability | UV sensitive unless buried | Stronger, longer lifespan |
12. Relying on DIY Instead of a Licensed Installer
What goes wrong:
Too many property owners try to save money by doing it themselves or using unlicensed tradies. Septic systems are not a weekend warrior job.
Why it matters:
Incorrect installation can void warranties, breach regulations, and cause pollution. A failing system isn’t just an inconvenience—it’s a serious liability.
How to avoid:
Hire a licensed wastewater plumber
Request credentials and insurance
Ask for references or past project photos
13. Not Registering Your System for Warranty and Council Records
What goes wrong:
Many homeowners don’t register their tanks with the supplier or local authorities, which creates long-term compliance issues and makes servicing harder.
Why it matters:
If something fails and you haven’t registered the tank or kept records, you may be up for the full repair cost—even within warranty.
How to avoid:
Register your tank with the supplier immediately (e.g., Eco Septic warranty form)
Notify council once installation is complete
Keep all documentation in a safe, easily accessible folder
Install Smart, Not Cheap
Installing a septic tank—whether concrete or poly—is a serious project that requires thoughtful planning, correct processes, and attention to detail. Cutting corners or skipping essential steps may seem like a time-saver, but it often ends in disaster.
Take it from the experts: it’s far cheaper to do it right the first time than to repair a system that’s failed due to poor installation. Engage a licensed installer, follow manufacturer recommendations, and always comply with council and environmental requirements.
At Eco Septic, we’ve seen the worst of the worst—and helped property owners fix what could have been easily avoided.
Need Help?
Whether you’re just planning your installation or fixing a failed one, we’re here to help. Contact Eco Septic today for advice on or recommendations:
Professional installation
Custom tank sizing
Council-compliant designs
Poly and concrete tank systems
Full after sales service and support
Call us on 1800 808 135 or visit www.ecoseptic.com.au
