Flooding on the Hastings River, Camden Haven, and the coastal tributaries of the Mid North Coast is a recurring reality. Properties at Wauchope, North Haven, Dunbogan, Port Macquarie flats, and rural blocks along the Hastings and Wilson rivers have experienced multiple flood events. If your septic system was inundated, you need to know what to check, when the system is safe to use, and when to call for professional assessment.
Do not assume a flooded septic system is fine just because the water has receded. Flood damage to onsite sewage systems ranges from temporary operational disruption to permanent structural failure, and using a damaged system before it’s assessed carries health and environmental risks.
What Happens to a Septic System During a Flood?
Understanding the failure modes helps you assess what you’re dealing with:
Inundated Absorption / Leach Field
The most common and most serious flood impact. When floodwater saturates the ground around and above the leach field:
- Soil pores fill with water, eliminating the capacity to absorb any additional effluent
- Floodwater can infiltrate the gravel bed and perforated pipe of the absorption trench, carrying sediment that partially or permanently seals the soil
- As floodwater recedes, it leaves a sediment layer on the soil surface that dramatically reduces future percolation capacity
Tank Flooding (Ingress of Floodwater)
If floodwater reaches the tank lid and the lid seal is not watertight, or if the tank is an older concrete tank with cracks, floodwater can enter the tank:
- Diluting the contents (temporarily reduces treatment effectiveness)
- Potentially floating the tank if it’s a lighter fibreglass or poly tank (uncommon but possible for empty or near-empty tanks)
- Introducing sediment that accumulates above the existing sludge layer
Tank Floatation
A very significant issue for fibreglass and poly tanks that are not anchored. An empty or recently pumped fibreglass tank has positive buoyancy in saturated, waterlogged soil. Tanks have been known to partially or fully “float” out of the ground during extreme flood events, damaging inlet and outlet connections and requiring complete reinstatement.
AWTS Submersion
If an AWTS unit is submerged in floodwater:
- The aerator and other electrical components can be damaged or destroyed
- Control panels and alarm systems are typically not waterproof at sustained submersion depth
- The drip irrigation area becomes completely non-functional while saturated
- On-panel alarms will activate (if power is maintained), follow guidance in our AWTS alarm guide
Pipe Displacement
The pipes connecting the house to the tank, and the tank to the land application area, can shift in flood-saturated soil, particularly in sandy or alluvial soils common in the Wauchope and Port Macquarie flats areas. Displaced pipes create blockages or open joints that allow untreated effluent to surface.
Immediate Steps After a Flood
During active flooding or floodwater still on the property:
- Do not use the septic system (flush toilets, run taps, use washing machine) if you can avoid it at all. Every litre you add to a system with a saturated dispersal area risks backup into the house.
- If you must use facilities during extended flood events, water use minimisation is critical.
- If you have an AWTS, turn off the power to the unit if it is submerged to prevent electrical damage to the pump and aerator while underwater.
Immediately after floodwaters recede:
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Visual inspection of the surface: Walk the land application area and check for surface breakout, effluent pooling, sewage smell, or unusually wet patches in the absorption area. Do not touch or walk through any area showing surface sewage.
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Check the tank: Look at the lid, is it still sealed? Is it sitting correctly? For poly/fibreglass tanks, is the tank still level or has it shifted?
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Check AWTS control panel: If you have an AWTS, check whether the alarm is active. If the unit was submerged, do not restart the power until the unit has been inspected by your service provider.
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Do not use the system for at least 24-48 hours after floodwaters recede: The soil needs time to begin draining. Running wastewater into the system while the ground is still saturated will cause immediate backup.
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Call for professional assessment if any concerns: A septic system inspection post-flood gives you a definitive picture of what, if anything, is damaged.
Post-Flood Assessment Checklist
| Item | What to Check | Good Sign | Concern |
|---|---|---|---|
| Tank lid | Seated, sealed | Lid in place, no debris | Lid displaced, cracked, or open |
| Tank level | Normal operating level | Clear water visible at inspection port | Tank overfull or filled with muddy water |
| Tank position (poly/fibreglass) | Tank still in original position | Tank level and connections intact | Tank shifted, connections broken |
| Land application area | Soil surface condition | No surface pooling 48 hrs after flood | Surface effluent, odour, ponding |
| AWTS control panel | Alarm status | No alarm, unit running normally | Alarm active, no power, control panel damaged |
| Drip irrigation | Lines visible/accessible | Lines intact | Lines displaced or silted |
| Inlet/outlet pipes | Connection integrity | Drainage flowing correctly when tested | Backup into fixtures, gurgling |
| Surrounding area | No contamination spread | No sewage odour outside system footprint | Sewage odour in yard or nearby waterway |
When Can You Use the System Normally Again?
Only when:
- Floodwaters have fully receded and the land application area has been dry for at least 48-72 hours
- Visual inspection shows no surface breakout
- Drainage from the house is flowing through the system without backup
- For AWTS: the unit has been inspected and restarted by your accredited service provider
- There are no signs of tank displacement, pipe damage, or structural failure
If you are in any doubt, have the system professionally inspected before returning to full use.
Post-Flood Pump-Out: Is It Necessary?
If the tank has been inundated with floodwater, a pump-out after the flood is good practice for two reasons:
- It removes flood-contaminated water and resets the biological treatment in the tank
- It allows inspection of the tank interior for sediment accumulation, structural damage, and baffle condition
A pump-out immediately post-flood, when the ground is still saturated, does carry risk for fibreglass tanks (reduced ground pressure holding the tank down). Discuss timing with your contractor.
We take enquiries for post-flood pump-outs and inspections across the Port Macquarie and Hastings region and can advise on the appropriate timing.
Long-Term Damage to Absorption Areas
Absorption trench damage from flooding is often not apparent until weeks or months after the flood. As the sediment deposited during inundation dries and consolidates, it can create a less-permeable layer in the soil, progressively reducing absorption capacity. Signs emerge when:
- Drains start backing up slowly in periods of heavy use
- Wet patches appear in the absorption area after normal use (not just during rain)
- AWTS high-level alarms activate when they didn’t before the flood
If you notice these signs in the months following a flood, book an inspection, catching absorption area degradation early gives you more options for remediation.
Insurance Considerations
Some contents and home insurance policies cover flood damage to permanently-installed property systems. Check your policy to see whether the septic system and associated infrastructure is covered, and what documentation is required for a claim. Post-flood inspection reports from a qualified contractor are typically required as supporting documentation.
FAQ
Can I flush the toilet during a flood? Only as absolutely necessary. Each flush adds 6-9 litres to a system whose dispersal area cannot accept any more water. In a prolonged flood event, water use minimisation is the most important thing you can do to prevent sewage backup into the house.
After the flood, my septic alarm went off and won’t stop, what does it mean? For AWTS systems, the alarm most likely indicates a high water level in one of the chambers, caused by floodwater ingress or an inability to disperse effluent to the saturated ground. Do not restart the AWTS aerator until the system has been inspected. Send an urgent enquiry via our emergency septic page.
The flood deposited mud over my absorption trench area, does that permanently damage it? Not necessarily permanently, but possibly. A single flood event’s sediment layer may only cause temporary reduction in surface permeability. Multiple floods or a very thick sediment layer can cause significant long-term capacity loss. An inspection several weeks after the flood event (when the soil has dried) is the best way to assess the extent of damage.
My poly tank appears to have shifted during the flood, is this an emergency? Yes, treat it as an emergency. A shifted tank may have disconnected inlet or outlet pipes, meaning sewage could be escaping into the surrounding soil uncontrolled, or backing up into the house. Call for immediate inspection.
Does flood damage to a septic system need to be reported to Council? If your system is discharging effluent to a waterway as a result of flood damage, this is an environmental incident that should be reported to the EPA and potentially Council. If the damage is contained within your property and the system is simply non-functional rather than actively discharging, formal reporting is not typically required, but getting it repaired quickly is both practical and responsible.
If your septic system was flooded and you need assessment, pump-out, or emergency assistance, send an enquiry. Enquiries come from across Wauchope, North Haven, Port Macquarie and the broader Hastings region, where flood risk is a lived reality.