An overflow relief gully is the small grated drain in the ground outside your home, usually near the bathroom, kitchen, or laundry. It’s the release point for your sewer pipes. If a sewer pipe blocks, wastewater spills out of the gully in your yard instead of coming up through your shower or floor waste inside.
It’s a simple part that does a big job, but only if it’s installed at the right height and kept clear. As part of our drainage work across the Inner West, we often find gullies that are buried, covered, or sitting too low to work. Here’s what yours does, the NSW rules it has to meet, and how to check it.
What is an overflow relief gully?
An overflow relief gully, often shortened to ORG or called a gully trap, is a vertical pipe connected to your private sewer drain. It finishes just above the ground with a loose grate on top. Under the grate, a bend in the pipe holds a small pool of water, called the water seal.
You’ll usually find it:
- against an outside wall near the bathroom or laundry
- in a courtyard or side passage
- under a tap fixed to the wall just above it
Every home connected to the sewer should have at least one. It’s part of your private plumbing, so like the rest of your sewer pipes, it’s yours to look after.
What does an overflow relief gully do?
Wastewater always takes the easiest way out. If the sewer pipe blocks, whether in your own drain or in the main in the street, the wastewater backs up and rises until it finds the lowest opening.
The gully is designed to be that lowest opening. Because it sits lower than every fixture in the house, wastewater reaches it first. The grate lifts off, and the overflow spills outside where you can see it, instead of flooding your bathroom.
That’s why the grate must sit loose. If it’s buried, covered, or stuck down, the water keeps rising and finds the next lowest point, which is usually your shower or floor waste.
Why is there a tap above it?
Many Inner West homes have a garden tap fixed to the wall just above the gully. It isn’t there by accident.
The water seal in the gully stops sewer smells coming up out of the drain. In older homes, water from fixtures such as the laundry tub used to flow over the gully and keep that seal topped up. Today, many homes no longer have anything draining into the gully, so in dry, hot, or windy weather the water slowly evaporates. Once the seal dries out, sewer gas escapes and you notice the smell.
The tap fixes that. Lift the grate, run the tap for a few seconds, and the seal is topped up again. It’s also handy for rinsing the gully clean.
Just don’t leave a hose connected and running into the gully, and don’t build anything around the tap that stops the grate lifting freely.
NSW height rules for overflow relief gullies
In June 2026, Building Commission NSW issued an advisory note on overflow relief gullies after its inspectors found non-compliant installations across the state. The note warns that a gully without the right height clearances can lead to significant property damage if the sewer surcharges or blocks.
In plain terms, the rules are:
- 150 mm below the lowest fixture: the top of the gully must sit at least 150 mm lower than the lowest fixture connected to the drain. For a shower or floor waste, that’s measured to the top of its grate.
- 75 mm above the ground: the top of the gully must sit at least 75 mm above the finished ground around it, so rain and surface water can’t flow in.
- paths and paved areas: where the gully sits in a path or paving, it must be finished so water can’t pond around it or run into it.
Where these heights can’t be achieved, NSW guidance calls for a reflux valve instead. This is a one-way valve that stops wastewater flowing back towards the house.
Renovations are where gullies go wrong
The biggest risk to a gully is usually a renovation. NSW Fair Trading noticed a sharp rise in installations that didn’t meet the height rules as designers worked to reduce the step between indoor and outdoor living areas.
It’s easy to see how it happens. A raised courtyard, a new deck, a garden bed, or fresh paving lifts the ground level around the gully, and suddenly it’s too low. Rainwater pours in during every storm, or the gully is buried altogether.
The new advisory note asks plumbers to consider future landscaping and pathways, and to confirm final ground and slab levels with the builder before work starts. If you’re planning a courtyard renovation in Newtown, Rozelle, or anywhere with tight terrace yards, talk to your plumber before the pavers go down, not after.
Common overflow relief gully problems
These are the problems we see most often:
- buried under a garden bed, mulch, or paving
- covered by a doormat, outdoor furniture, or a pot plant
- cracked or broken around the top
- downpipe draining into it, which is not allowed
- grate concreted in place, corroded, or stuck down
- too low, so stormwater flows straight in
Several of these cause a second problem. Sydney Water says a damaged gully or one lower than the ground is one of the ways stormwater gets into the sewer, along with downpipes connected to it. During heavy rain, that extra water overloads the system and can cause the very overflows the gully is meant to protect you from.
The fix is usually simple. Hornsby Council notes that a plumber can raise a low-lying gully or lower the ground around it.
How to check your overflow relief gully
It takes five minutes:
- Find the gully outside, usually near the bathroom, kitchen, or laundry.
- Clear away anything sitting on or around it.
- Lift the grate gently. It should come away easily, without tools.
- Look inside. You should see water sitting in the bottom of the trap.
- If it’s dry or smells, run the tap above it for a few seconds.
- Check that it sits above the surrounding ground and clearly lower than your shower floor.
- Make sure no downpipe or stormwater drain empties into it.
If the grate is stuck, the gully is cracked, or it sits at or below the ground, it’s time to call a plumber.
What to do if your gully overflows
Wastewater coming out of your gully means the sewer is blocked somewhere downstream. Stop using water in the house straight away, because every flush and shower adds to the overflow. Keep children and pets away from the area, and avoid touching the wastewater.
Then call a licensed plumber for emergency plumbing help to find and clear the blocked drain. If the blockage keeps coming back, a CCTV drain inspection can show what’s causing it. If you notice wastewater overflowing in the street or a public area, report it to Sydney Water on 13 20 90.
Frequently asked questions
Why does my gully smell?
Usually because the water seal has dried out. Run the tap above it for a few seconds to top it up. If the smell keeps coming back, the gully or the pipe below it may be damaged, so have a plumber check it.
Why does my gully overflow when it rains?
Rainwater is probably getting into your sewer pipes. The gully may be cracked, have a downpipe draining into it, or sit too low. A plumber can find the cause and stop the stormwater getting in.
Can I cover my gully with pavers or a pot plant?
No. The grate must stay uncovered and loose so it can lift off during an overflow. If it’s covered, wastewater may come up inside your home instead.
Can I raise or move my overflow relief gully?
Yes, but it’s work for a licensed plumber. A gully can usually be raised to the right height, or the ground around it lowered. Where the height rules can’t be met, a reflux valve may be needed.
Book an overflow relief gully check
Richard Knight Plumbing has been looking after Inner West drains from our base on Raper Street, Newtown, since 2005. We can check your gully’s height and condition, raise or repair it, stop stormwater getting in, and make sure it will do its job when you need it.
We work Monday to Friday, 7 am to 5 pm, across all the suburbs on our service area page. Call 1300 300 941 or request a quote, and we’ll get back to you with clear advice and an obligation-free quote.