Septic tanks are full of untreated wastewater which if abandoned and incorrectly decommissioned, can leak contaminants into the soil and groundwater. Find out about requirements for decommissioning a septic tank.
Why do septic tanks need to be decommissioned?
Septic tanks are full of untreated wastewater. Wastewater is loaded with contaminants including bacteria, viruses, parasites and nitrates which can cause diseases or other health or environmental problems. If abandoned and not properly decommissioned, wastewater will eventually leak out, contaminating the soil and groundwater and polluting local watercourses.
Septic tanks that have not been properly taken out of service can also create a safety hazard. If an abandoned septic tank has not been filled in with soil, the lid or top of the tank may collapse or cave in. There have been serious injuries and even deaths caused by a fall into an abandoned septic tank when the lid collapsed.
When do you need to decommission a septic tank?
There is no legal obligation to have an unused septic tanks system decommissioned at the time of sewer connection, unless one of the below situations exists: 
  • If a property is sold, the new owner has a legal obligation under the Health (Treatment of Sewage and Disposal of Effluent and Liquid Waste) Regulations 1974, to decommission the system within 60 days after settlement of the property.
  • If building extensions encroach on minimum clearances from the system. Septic tanks must be located at least 1.2 metres from any building or boundary and leach drains/soakwells must be located 1.8 metres from any building or boundary.
  • If the permitted use of the development changes e.g. from a residence to a commercial use.
However, consideration should be given to decommissioning septic systems at the earliest possible convenience, as the drainage receptacles may become unstable over time, which can cause them to collapse.

What does decommissioning involve?
When decommissioning, only a registered liquid waste contractor is to be used.
The steps involved are:
  1. Empty all septic tanks, soakwells, leach drains or other on-site disposal systems.
  2. Remove the tanks, or if impractical, break the base of the tank.
  3. Backfilling all tanks, soakwells and leach drains with clean fill.
  4. Keep the receipt from the contractor to prove that decommissioning has been completed.


As the original system may have been extended by the addition of extra soakwells or leach drains, it is important to ensure that the whole of the system is located. For further enquiries please contact the City of Stirling on (08) 9205 8555.