TL;DR
- An overflow relief gully (ORG) overflowing once might be the system doing its job. Overflowing repeatedly means there’s a problem that needs fixing.
- The most common causes are a blockage in the sewer line, tree roots in the pipe, a collapsed or damaged pipe, or a sewer network surcharge during heavy rain.
- Stop running water in the house as soon as you see sewage coming up through the gully. Every flush and tap makes it worse.
- Clearing the gully grate yourself is fine. Everything beyond that — clearing the sewer line, inspecting the pipe, fixing a structural fault — is licensed work in NSW.
- If it keeps coming back after clearing, a CCTV inspection is the only way to find out why.
The ORG overflowed. Is that actually a problem?
One overflow during a severe storm, or right after a major sewer blockage gets cleared, isn’t necessarily alarming. The overflow relief gully is designed to release pressure from the sewer line before that pressure finds its way inside your home. In that sense, it’s doing exactly what it’s supposed to do.
But if it’s overflowing regularly — after showers, during laundry cycles, whenever the household uses water heavily — that’s a different situation. The system is telling you something is wrong downstream, and it’s not going to fix itself.
The distinction matters: a one-off overflow is a symptom. A recurring overflow is a fault.
What the overflow is actually telling you
The ORG sits lower than every indoor drain in your home. That height difference is deliberate — it means when the sewer line backs up, wastewater has a release point outside rather than coming up through your shower floor or toilet.
When sewage rises through the gully grate, it means the pipe running from your home to the sewer junction is blocked, restricted, or damaged badly enough that water can’t get through. The household waste has nowhere to go, so it rises back up and escapes at the lowest available point — the ORG.
That’s useful information. It tells you the problem is in the sewer line, not a single fixture. And it tells you the pressure is already high enough to push waste back toward your property.
The five causes worth knowing about
1. A blockage in the sewer line
The most common cause. Grease, wet wipes, sanitary products, and accumulated debris build up in the pipe over time. At some point the restriction becomes severe enough that normal household water use can’t push through it. The ORG overflows as a result.
This type of blockage is often gradual — drains slow down over weeks before the gully starts overflowing. If you’ve noticed slower drains across the house recently, that pattern matters.
2. Tree roots in the pipe
Tree roots are the most common cause of recurring sewer blockages in older Sydney suburbs. Roots seek moisture and find their way into clay pipes through joints and small cracks. Once inside, they grow into a mass that catches everything passing through the line.
The problem with roots is that clearing the blockage doesn’t remove the root mass — it just cuts through it temporarily. If your ORG keeps overflowing every few months after being cleared, roots are the likely reason. The only permanent fix is either relining the pipe or replacing the affected section.
If you want more detail on why this pattern repeats, the post on drains that keep blocking after being cleared covers the root cause in depth.
3. A structural fault in the pipe
Clay sewer pipes in older Sydney homes crack, collapse, and develop belly sections — low spots where water pools instead of flowing through. A collapsed pipe or a significant belly creates a collection point for waste. The pipe can’t move water efficiently, and eventually the ORG overflows under normal household load.
This kind of fault won’t respond to clearing. The blockage comes back because the pipe itself is the problem, not what’s inside it. A CCTV camera inspection is the only reliable way to confirm this.
4. Sewer network surcharge during heavy rain
Sometimes the overflow isn’t caused by anything on your property at all. During heavy rain events, the wider sewer network can become overwhelmed. That pressure pushes back toward private properties, and the ORG overflows as a result.
If the overflow only happens during or immediately after significant rainfall — and your drains run normally at all other times — this is the likely explanation. In that case, the fault may sit with Sydney Water rather than on your side of the boundary. You can report it to Sydney Water on 13 20 90.
That said, a cross-connection between your stormwater and sewer system can make rain-triggered overflows much worse. If stormwater is entering the sewer line, the volume during rain events can be far higher than the network can handle. This is worth investigating if the rain-triggered overflows are frequent or severe.
5. The ORG itself is blocked or obstructed
Less common, but worth checking first. Leaves, garden debris, and soil can block the gully grate or accumulate in the trap body. If the grate is blocked, the ORG can’t do its job — pressure that would normally escape outside instead builds up in the system.
Pot plants placed over the gully, garden beds built around it, or paving laid on top of it are all common problems in Sydney homes. The gully needs to be clear and accessible at all times.
What to do right now if it’s overflowing
Stop running water in the house. That means no flushing, no showers, no washing machine, no dishwasher. Every litre you send down the drain adds to the pressure that’s already backing up. The overflow will get worse, and you increase the risk of sewage getting inside.
Clear any leaves or debris from the gully grate — you can do that yourself. Beyond that, don’t try to clear the sewer line with a hose or by pouring anything down the gully. You won’t fix a sewer blockage that way, and you risk making the overflow worse.
Call a licensed plumber. A sewer line blockage needs either a drain machine or a high-pressure water jetter to clear properly, and that’s licensed work in NSW.
What you can check yourself (and where to stop)
There are a few things worth doing before the plumber arrives, as long as you stay safe around sewage:
- Check the gully grate. Remove any leaves, soil, or debris sitting on top of or inside the grate. This takes two minutes and rules out the simplest cause.
- Check whether other drains in the house are also slow. If every fixture is sluggish — not just one — the blockage is in the main sewer line, not a single branch pipe.
- Check whether the overflow is happening during rain or under normal use. This tells you whether the cause is likely on your property or in the wider network.
- Note how often it’s been happening. A plumber will ask. Monthly recurrence points to roots or a structural fault. A single event after a storm points somewhere else entirely.
Don’t attempt to clear the sewer line yourself, and don’t put chemicals down the gully. Sewer work is licensed work in NSW, and chemicals won’t shift a root mass or a collapsed pipe section.
Why recurring overflows point to something more serious
A one-time overflow that clears after a plumber jets the line is one thing. An ORG that overflows again three months later — then again two months after that — is telling you the clearing isn’t solving the underlying problem.
The two most likely explanations for a recurring overflow are tree roots regrowing in a damaged pipe, or a structural defect that creates a collection point. Neither of these goes away with repeated clearing. They need to be diagnosed properly.
A CCTV drain inspection is the right tool here. The camera goes into the pipe and shows exactly what’s happening — root intrusion, a crack, a belly, a collapsed section. Once you know what you’re dealing with, the fix is usually straightforward: relining, targeted replacement, or in some cases just a change in maintenance frequency.
For more on what a damaged underground sewer pipe looks like from the surface, the post on signs of a damaged underground sewer pipe is worth reading before you call.
What a plumber actually does when the ORG keeps overflowing
A licensed plumber will start by assessing the gully and running a quick check on the drainage pattern — which fixtures are slow, whether the overflow is tied to specific water use, how frequently it’s been happening.
From there, the approach depends on what they find:
- If it’s a straightforward blockage: a drain machine or water jetter clears the line. The plumber will usually recommend a CCTV check if the blockage pattern suggests roots or structural damage.
- If the CCTV shows roots: the options are pipe relining (a resin liner inserted inside the existing pipe to seal cracks and block root entry) or pipe replacement for more severely damaged sections.
- If the CCTV shows a collapse or belly: the affected section needs to be replaced. The plumber will advise on the extent of the work and what access it requires.
- If the problem is a cross-connection: stormwater entry into the sewer line needs to be identified and rectified, which is regulated work under NSW plumbing standards.
All of this is licensed work. The certificate of compliance issued after any sewer repair is your record that the work was done to the required standard.
A note on older Sydney homes
If your home was built before the 1980s, the sewer line is almost certainly clay. Clay pipes are prone to cracking as they age, and Sydney’s tree canopy — particularly in inner and middle-ring suburbs — means root intrusion is a near-universal problem in older properties.
An ORG that overflows in an older home is more likely to be pointing at a structural pipe fault than a simple grease blockage. That doesn’t mean the job is necessarily expensive, but it does mean a CCTV inspection is usually worth doing before committing to repeated clearing visits that won’t address the root cause.
For a broader look at what the ORG is, how it’s positioned relative to your indoor fixtures, and what NSW requires, the post on what an overflow relief gully is and why your Sydney home has one covers the fundamentals.
When to call straight away
Call a plumber today — not next week — if:
- Sewage is coming up through the gully and you can’t stop it by turning off the water
- Sewage is backing up inside the house through floor wastes, the shower, or the toilet
- The overflow is happening under light water use — a single flush, a short shower
- There’s a sewage smell inside the house alongside the gully overflow
- The overflow has happened more than once in the past few months
Any of those situations means the sewer line is compromised enough that normal household use is a risk. Stop using water and get someone out.
Jasco Plumbing handles blocked sewer lines and CCTV drain inspections across Sydney. If your ORG keeps coming back, tell us what’s been happening and we’ll give you a straight answer on what’s causing it and what it’ll take to fix it properly.