What is a septic tank? How it works, and what to ask before you rely on one
No sewer connection means the wastewater is treated on your block. Here is how that works, what goes wrong and who has to do the work.
A septic tank is a buried, watertight tank that treats the wastewater from a house that is not connected to the town sewer. Solids settle, scum floats. And the liquid in the middle drains out to a disposal area in the ground. It is common on rural and some fringe-suburban blocks across Australia.
If you have bought or rent such a property, knowing what the system does helps you ask better questions. The installation and repair work is licensed, and the rules are set by each state and territory.
How does a septic tank work?
All wastewater from toilets, showers, basins and the laundry runs into the tank. Inside, it separates into three layers. Heavy solids sink to form sludge, fats and light material float as scum, and the clearer liquid sits between them.
Bacteria in the tank break down part of the solids. The liquid leaves through an outlet and goes to a trench, bed or other disposal area, where the soil does the final treatment. The soil is half the system. A perfect tank on the wrong soil still fails.
| Part | What it does |
|---|---|
| Inlet pipe | Brings wastewater from the house into the tank |
| Tank | Holds the water long enough for solids and scum to separate |
| Outlet and filter | Lets the middle layer out and keeps solids back |
| Disposal area | Spreads the liquid into soil for final treatment |
| Lid or inspection opening | Gives access for pump-outs and inspection |
Septic tank or sewer: how do you tell which you have?
Look for a lid or inspection opening in the yard, and check your property records or council file. A house with no sewer connection point near the boundary is a clue. Do not guess: ask the seller, your council or your plumbing regulator to confirm what the property has.
Many buyers assume a house is on the sewer and find out late. Make it one of your first questions.
What goes wrong with a septic system?
- The tank fills with sludge. It needs emptying by a licensed contractor at the interval your regulator or the system manual gives.
- The disposal area clogs. Wet, smelly ground over the trench is a warning sign.
- Slow drains and gurgling throughout the house can mean the tank or outlet is backing up.
- Harsh chemicals and non-flushable items upset the bacteria and block the system.
Plenty of problems start with the household. Keep fats, wipes and strong cleaners out of the drains.
What should you ask about a septic tank?
- Is the system approved by the council or regulator, and where is the paperwork?
- When was it last emptied and inspected, and is there a record?
- Where is the disposal area, and is it kept clear of vehicles and deep-rooted trees?
- Does your council require regular inspection or a service agreement?
Is a septic tank the same as other onsite systems?
No. A traditional septic tank relies on gravity and soil. Other onsite systems add aerators, pumps or extra treatment stages before the water leaves the block. They need more electricity and more regular servicing, and many states require a service agreement for them.
If the property has anything with a switchboard, an alarm or a pump chamber, treat it as a more complex system. Ask for the manual and the service history before you commit.
Is it worth keeping a septic system running well?
Yes, and it is cheaper to look after than to replace. A tank that is emptied on schedule and a disposal area that is kept clear can last for many years. A neglected one can foul the soil and contaminate groundwater, and a failed disposal area is often the expensive part of a repair.
If a property is on septic, ask about the system before you ask about the kitchen. The answer can change what the place is worth to you.
How far can you take this yourself?
Finding the lid, keeping a record of pump-outs and watching the ground for wet patches are all yours. Choosing what goes down the drain matters most of all.
This far, and no further. Installing, altering or repairing a septic system is licensed work, and many states also require council or health approval. Never open a tank: the gases inside can be fatal. Contact your state or territory plumbing regulator and your local council to find out what applies at your address. Our guide to what plumbing you can do yourself shows how to check.
Frequently asked questions
What is a septic tank?
It is a buried, watertight tank that treats household wastewater where there is no sewer connection. Solids settle and the liquid drains to a disposal area in the soil.
How do I know if my house has a septic tank?
Look for a lid in the yard and check your property records or council file. If you are unsure, ask your council or plumbing regulator to confirm.
Can I fix a septic tank myself?
No. Installation, alteration and repair are licensed work, and opening a tank is dangerous because of the gases inside. Call a licensed plumber.
What damages a septic system?
Overfull tanks, a clogged disposal area, fats, wipes and harsh chemicals. Regular emptying by a licensed contractor prevents most failures.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NSW Fair Trading. Plumbers, drainers and gasfitters (NSW Fair Trading)
- Australian Building Codes Board. National Construction Code (Plumbing Code of Australia)