Decommissioning a septic tank is required when a property connects to town sewer, when an old tank is being replaced with a new system, or when a tank has failed and is being permanently taken out of service. An old tank that is simply abandoned in place without formal decommissioning is a safety hazard, the roof of a concrete tank degrades over decades and can collapse without warning, creating a hidden void in the ground.
In NSW, decommissioning must be done by a licensed plumber and drainer, typically requires notification to Port Macquarie Hastings Council, and follows a defined process to make the tank safe and remove it from the Council’s onsite sewage management register.
When Decommissioning Is Required
You need to decommission an old septic tank when:
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Connecting to town sewer: When the property connects to the reticulated sewer network, the existing septic system must be decommissioned. Council will require evidence of decommissioning.
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Replacing with a new system: When an old failed tank is replaced by a new tank (or a new AWTS), the old tank must be decommissioned rather than left in place, unless it can be repurposed (see below).
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Subdividing land: When land is subdivided, any septic tanks on the lot must be properly decommissioned or transferred with the lot in a registered state.
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Building over or near the old tank: If development requires building within the old tank’s footprint or within proximity where a void would create a foundation risk, the tank must be decommissioned and verified as structurally sound before building.
NSW Regulatory Requirements
Under the Plumbing and Drainage Act 2011, all plumbing and drainage work, including the connection work to sewer and the decommissioning of existing systems, must be carried out by a licensed plumber and drainer. The work must be notified to the relevant certifying authority.
The local council (Port Macquarie Hastings Council in this region) administers the onsite sewage management register. Decommissioning is the mechanism for removing a property from the OSM register when it is no longer needed.
There is no separate “decommissioning permit” in the DA sense, the work is typically carried out under a plumbing works approval and notified to Council on completion.
The Decommissioning Process Step by Step
Step 1: Pump Out the Tank
Before any other work, the tank must be pumped out by a licensed liquid waste contractor. This removes all remaining sewage solids, sludge, and liquid from the tank. The waste is transported to an approved disposal facility.
Do not skip this step or attempt to collapse the tank with waste still inside, this is an environmental offence and creates a significant contamination risk.
We take enquiries for septic pump-outs and tank cleaning as the first stage of decommissioning across the Port Macquarie, Wauchope, and Hastings region.
Step 2: Clean and Disinfect the Empty Tank
After pump-out, the inside of the tank is cleaned and disinfected. This removes biofilm, residual sludge, and pathogens from the tank walls and floor. Cleaning is typically done by high-pressure water with a disinfectant such as lime wash or diluted bleach solution.
Step 3: Disconnect and Cap All Pipes
The inlet and outlet pipes connecting the tank to the house drainage system and to the land application area are cut and capped off at or near the tank. The sewer connection to the house must be properly redirected to the new sewer connection point (for connections to town sewer) or to the new system.
Step 4: Break Up or Fill the Tank
Options for the old tank structure depend on the tank type and site requirements:
For concrete tanks:
- The roof (top slab) is broken up and pushed into the tank cavity. The tank is then filled with clean fill material such as sand, gravel, or certified fill, and compacted.
- Alternatively, the entire tank (if accessible by excavator) can be removed from the ground, broken up, and removed from site.
- Breaking the roof prevents the future collapse risk of an empty underground void.
For fibreglass and poly tanks:
- These cannot simply be filled and left, the tank will deform or pop up over time if there is groundwater pressure. They must be removed from the ground.
- Smaller fibreglass tanks can be cut out and removed. Larger tanks may need to be broken up in-situ.
For brick/masonry tanks (older properties):
- Similar to concrete, break down the crown and fill with clean material. Solid masonry tanks rarely need full removal if structurally sound and properly filled.
Step 5: Restore the Surface
Once filled, the site is restored with topsoil and stabilised to prevent erosion. For areas that will be used as lawn, garden, or light trafficable areas, the fill should be compacted to prevent future subsidence.
Step 6: Notify Council
The licensed plumber or certifier notifies Port Macquarie Hastings Council of the completed decommissioning. Council removes the property from the OSM register, or updates the record to show the new system in place.
Can the Old Tank Be Repurposed?
In some situations, an old septic tank that is structurally sound can be repurposed rather than demolished and filled:
Stormwater storage or grey water holding: An old concrete tank can sometimes be converted to a stormwater detention or irrigation storage tank, provided it is properly cleaned, disinfected, and any old sewage connections are completely separated. This requires plumbing approval.
Garden water tank: On rural properties, a large old concrete tank (3,000-5,000 L) can sometimes be repurposed as a garden water storage tank. Again, this requires appropriate cleaning and Council notification.
Not for any potable (drinking) water use: No matter how well cleaned, a tank formerly used for sewage must never be repurposed for drinking water storage.
Costs for Septic Tank Decommissioning (2026 Mid North Coast)
| Work Component | Typical Cost (AUD) |
|---|---|
| Pump-out of remaining contents | $300 to $600 |
| Tank cleaning and disinfection | $200 to $400 |
| Pipe disconnection and capping | $300 to $600 |
| Concrete tank: roof demolition and fill | $800 to $2,000 |
| Concrete tank: full excavation and removal | $2,000 to $5,000 |
| Fibreglass/poly tank removal | $1,000 to $3,000 |
| Site restoration (topsoil and compact) | $300 to $800 |
| Total indicative range | $2,000 to $9,400 |
Costs vary significantly depending on tank size, depth of burial, site access, and whether full excavation or fill-in-place is used. Filling in-place (breaking the concrete roof and filling) is substantially cheaper than full excavation and removal, and is the standard approach for most concrete tanks.
Decommissioning as Part of Sewer Connection
When a property in Port Macquarie connects to town sewer, the Council typically requires evidence that the existing septic has been decommissioned as a condition of the sewer connection approval. The licensed plumber who makes the sewer connection is usually the same contractor who decommissions the old tank, a logical sequencing that minimises the disruption to the property.
Disclosure on Property Sale
If you are selling a property and have previously decommissioned a septic tank on it, note this clearly in your Section 10.7 certificate and conveyancing documents. A filled old tank is not a hazard if properly decommissioned, but buyers should be aware of its location, especially if building works are planned. The filled area will typically show as a dip or soft spot in the lawn for a few years until the fill settles.
See our guide on septic registration and change of ownership for more on disclosure and property sale requirements.
FAQ
Do I need Council permission to fill in an old septic tank? In NSW, decommissioning plumbing work requires a licensed plumber. The Council is typically notified as part of that work process, it is not a separate DA, but it does need to be properly done and recorded. Simply burying an old tank without proper pump-out, disinfection, and filling is not compliant.
Can I leave the old tank in the ground if I’m connecting to sewer? No, Council will typically require decommissioning of the existing septic as a condition of the sewer connection approval. A tank left in place without being properly decommissioned remains a safety risk and keeps the property technically on the OSM register.
My property has a concrete tank that was installed in the 1970s, how do I find it? Older tanks may not have inspection riser lids at the surface. Our septic inspection service includes locating buried tanks using probing rods and reference to property records. Council’s OSM register should also have a record if the tank was permitted.
Is there asbestos in old concrete septic tanks? Pre-1990 concrete products sometimes contain asbestos fibres, particularly in formed concrete products or cement-pipe systems. If you are demolishing or excavating an old concrete septic tank from before 1990, arrange an asbestos assessment first. An accredited asbestos assessor can test the concrete or advise based on the product type and date of manufacture.
How long does the decommissioning process take? For a straightforward fill-in-place decommissioning of a conventional concrete tank, the physical work takes half a day to a full day. Including pump-out, cleaning, and concrete roof demolition plus fill, allow 6-8 hours of on-site time.
Need to decommission an old septic tank in the Port Macquarie or Hastings region? Get a quote for pump-out, cleaning, and decommissioning, the operator handles the whole process from first pump-out to Council notification.