Every residential property on the Central Coast has at least two separate underground drainage systems, the sewer and the stormwater. They look similar from outside and run below the same garden, but they go to completely different destinations and have different failure modes. Working out which system has the problem is the critical first step in getting the right fix.
Quick answer (BLUF)
If your toilet, shower, sink or laundry backs up, it is the sewer system. If your garden floods in rain, downpipes are overflowing or a drain pit in the yard is full of water, it is likely the stormwater system. If you have a foul smell combined with wet ground, it could be a sewer leak rather than a blockage. Each system has different relining approaches and the failure symptoms are distinct once you know what to look for.
The two systems: what each carries
The sewer system carries all waste and wastewater from inside the house, toilets, showers, baths, kitchen sinks, laundry tubs and dishwashers. This water goes to the public sewage treatment network. The sewer system is pressurised only at the point of flush, otherwise it operates by gravity. Every fixture has a P-trap that creates a water seal against sewer gases.
The stormwater system carries surface water and rainwater, from roof downpipes, surface drains, driveway drainage, subsoil drainage and any landscape drainage installed around the property. Stormwater goes to the public stormwater network and eventually to a waterway or the ocean. In Central Coast terms, that is typically Brisbane Water, Tuggerah Lake, the Entrance channel or a coastal creek system.
The two systems must not cross-connect. Sewage entering the stormwater system causes environmental pollution and is a council compliance issue. Stormwater entering the sewer system increases the load on the treatment plant, also prohibited.
Identifying a sewer blockage
Sewer blockages affect the internal plumbing of the house. Signs include:
- Multiple fixtures backing up simultaneously: When you flush the toilet and water rises in the shower or bath, the blockage is in the main drain downstream of where those connections merge, a main sewer blockage
- Gurgling sounds from floor drains when the toilet is flushed, air displacement around a partial blockage
- Slow drainage across multiple fixtures, partial main sewer blockage
- Overflowing gully trap, the outdoor fitting (usually near the laundry or bathroom) that overflows when the main sewer is blocked, acting as a relief valve
- Sewage smell from internal drains or the ground around the sewer line
A single slow drain (just the kitchen sink, or just the bathroom basin) is more likely a blockage in that fixture’s individual branch pipe rather than the main sewer.
Identifying a stormwater blockage
Stormwater blockages affect the external drainage of the property. Signs include:
- Garden flooding during rain, particularly if it did not used to happen
- Downpipes overflowing at the base, the underground stormwater connection from the downpipe is blocked
- A stormwater pit or grate drain backing up with water ponding around it
- Water tracking along the surface toward the house, stormwater not going underground as intended
- Muddy water emerging from a drain pit after rain, silt-loaded water that should be flowing away underground
Stormwater problems are almost always weather-dependent. If the problem only occurs during or immediately after rain, it is stormwater.
When it might be a sewer leak rather than a blockage
This is an important distinction. A wet patch in the garden above the sewer line does not always mean a blockage, it can mean the sewer pipe is leaking, either from a cracked barrel or open joints, with effluent leaching into the soil. Signs of a sewer leak vs a blockage:
| Blockage | Leak |
|---|---|
| Internal drainage slow or backing up | Internal drainage normal |
| Gully trap overflowing | Gully trap not affected |
| Wet patch occurs when plumbing is used heavily | Wet patch present even when house is not in use |
| Smell from drains inside house | Smell from garden, not inside |
| CCTV shows obstruction in pipe | CCTV shows crack or open joint with clear pipe beyond |
A sewer leak is potentially a more serious environmental and public health issue than a blockage. It requires prompt attention even if the internal plumbing is currently working fine.
How to confirm which system: DIY test
You can do a rough diagnostic before calling a contractor:
For suspected sewer: Block off the gully trap inlet temporarily (a rubber stopper works) and flush the toilet. If water backs up faster with the gully trap sealed, the blockage is in the main sewer downstream of the gully trap.
For suspected stormwater: Run a hose into each downpipe inlet in turn, filling it with water. If a specific downpipe runs slowly or backs up while others clear, the underground stormwater line from that downpipe is blocked.
For a suspected leak: Turn off all water use in the house and check the water meter. If the meter is moving with everything turned off, you have a leak somewhere in the system (not necessarily the drain, it could be the supply side). If the meter is still, the issue is in the drain, not the supply.
What relining approach applies to each system
The relining technique is similar for both sewer and stormwater, an inverted or pulled-in liner, cured in place. But the materials, the access approach and the compliance requirements differ:
Sewer drain relining uses liners rated for continuous contact with waste and wastewater. The installation involves temporary sewer bypass for the duration of the job in some cases.
Stormwater drain relining uses liners compatible with surface water, sediment and in some coastal areas saline water. Access is typically through existing stormwater pits or new inspection openings.
If your drainage system has a cross-connection (stormwater pipes tied into the sewer, or vice versa), this needs to be identified and corrected as part of any relining project, not ignored.
FAQs
My toilet flushes fine but my garden is wet after rain. Which system is affected?
Almost certainly the stormwater system. If the sewer were failing, your toilet and other internal fixtures would be affected. A wet garden after rain indicates the stormwater pipe is blocked, broken or undersized for the current rainfall load.
Can one CCTV inspection check both systems?
Yes, but they require separate camera runs, the access points are different. Some contractors include both systems in a combined inspection report. This is worthwhile for older properties where both the sewer and stormwater are at similar ages and risk levels.
Who is responsible for the stormwater connection to the street?
Generally, the property owner is responsible for the private stormwater system from the house to the point where it connects to a Council drainage pit or kerb inlet. The Council is responsible for the public stormwater network from that connection point onward. If the Council’s infrastructure is causing your drainage problem, you can request Council inspection and repair.
Is it safe to flush the toilet if the main drain is partially blocked?
Avoid heavy water use until the blockage is cleared. Light toilet use may be fine short-term if drainage is slow but not completely stopped. Avoid running the dishwasher, laundry or multiple showers simultaneously, any large volume of water may cause a complete backup.