The overflow relief gully: the drain that protects your floors
The grate in the garden is the one fitting standing between a blocked sewer and your bathroom floor, and two measurements decide whether it works.
An overflow relief gully is the grated drain outside a house designed to release a sewer surcharge outside. Its overflow level must generally be at least 150mm below the lowest connected fixture and 75mm above the surrounding surface, with a specific exception for suitably graded paved or path areas. If it overflows, stop using water and get the drainage assessed.
Key takeaways
- The gully is a deliberate weak point in your drainage, designed so a surcharging sewer relieves outside rather than through the lowest drain in the house.
- Check the required clearances: normally at least 150mm below the lowest connected fixture and 75mm above the finished surface, with the paved or path-area exception explained below.
- Building Commission NSW issued an advisory note in June 2026 because inspectors kept finding gullies installed without those clearances.
- Landscaping is the common way a compliant gully stops being compliant, because new paving or a raised garden bed buries the riser.
- If it is overflowing, stop using water, keep the grate uncovered, and work out whether the blockage is yours or Sydney Water’s before anyone digs.
Jump to a section8
- What an overflow relief gully actually is
- Why an overflow relief gully matters more on a sloping North Shore block
- The two heights that decide whether an overflow relief gully works
- What to do when your overflow relief gully is overflowing
- Whose blockage is it, yours or Sydney Water's
- What it costs to find and clear the blockage behind an overflowing gully
- How to keep an overflow relief gully working
- Frequently asked questions
What an overflow relief gully actually is
An overflow relief gully is the round grate sitting at ground level somewhere outside your house, usually near an outside tap and often close to the bathroom or laundry wall. Sydney Water describes it as an open grid drain that lets a surcharging wastewater system relieve itself at that point first, rather than inside the building.
It is a deliberate weak point. When the sewer downstream of your house blocks, the wastewater backing up has to go somewhere, and the only question is whether it comes out of a grate in the garden or the lowest drain inside the house, which is usually the shower.
How to find yours:
- Look for a round grate raised slightly above the soil or paving, most often plastic or cast iron, with a removable grate on top.
- Check near an outside tap, near the laundry, or on the side of the house closest to the street.
- It is not the stormwater pit. Stormwater grates take rain off the roof and paving. The gully belongs to the sewer, so it will usually sit close to the house rather than out in the lawn.
If you cannot find one at all, that is worth knowing rather than ignoring. It means a blockage has no controlled place to surface.
Why an overflow relief gully matters more on a sloping North Shore block
Plenty of North Shore blocks fall away from the street, and the foreshore suburbs around Mosman, Northbridge and Hunters Hill are full of houses sitting well below the road they are addressed from. On a block like that, the gully is often the only fitting between a blocked sewer and a finished lower floor.
Three things about the area make this more than theory:
- Mature trees and old earthenware pipes. The upper North Shore canopy sits over sewer lines laid generations ago, and roots find the joints. That is the blockage the gully exists to catch.
- Landscaping raises the ground. Retaining walls, new turf, a raised garden bed or a repaved path can lift the finished ground level around a gully that was compliant when it went in.
- Falls are tight on a steep site. Where the house sits low, there is less height to play with between the drain, the gully and the lowest fixture inside.
The two heights that decide whether an overflow relief gully works
The heights are not a rule of thumb, they are set out in AS/NZS 3500.2:2026. Building Commission NSW issued an advisory note in June 2026 (Note 1/2026) precisely because its inspectors kept finding gullies installed without the required clearances, and it warns that failing to achieve them can cause significant property damage when a sewer surcharges.
| Clause | What it requires | Why it exists |
|---|---|---|
| 4.6.6.6 | At least 150mm between the top of the gully riser and the lowest fixture connected to the drain | Guarantees the gully overflows before the shower, floor waste or toilet inside does |
| 4.6.6.7 | At least 75mm between the top of the riser and the finished surface level | Stops surface water and garden runoff pouring into the sewer, and keeps the grate clear of soil |
| 4.6.6.7 exception | In a path or paved area, finished so water cannot pond or get in | Recognises that a raised riser in a path is a trip hazard, so the grading has to do the work instead |
Those two measurements are the whole mechanism. Lose the 150mm and the house becomes the overflow point. Lose the 75mm and the gully takes stormwater it was never meant to carry.
Where a site genuinely cannot achieve the heights, the standard allows a reflux valve instead, under Clause 4.5.2. It comes with its own condition: the Plumbing Code of Australia NSW variation prescribes that a sanitary drainage or waste stack is not installed upstream of the valve. That is a licensed plumber’s call, not a retrofit to attempt yourself.
Want this looked at rather than guessed at? Voyager agrees a fixed price before the work starts, and there is no callout fee on eligible jobs.
Call 0485 028 787 or read more about blocked drains on the North Shore.
What to do when your overflow relief gully is overflowing
An overflowing gully is doing its job. It is also telling you the line is blocked and the clock is running, because the next place the water rises is inside.
- Stop using water in the house. Every flush, shower and load of washing adds to what has nowhere to go.
- Keep people and pets away from it, and avoid tracking anything back indoors. Treat what comes out as contaminated.
- Never seal, cover or weigh down the grate. Blocking the relief point does not stop the surcharge, it just moves it inside.
- Work out whose problem it is using the signs in the next section.
- Call Sydney Water on 13 20 90 if it looks like their line, which they staff around the clock, or a licensed plumber if it looks like yours.
One practical note if you do call a plumber: where the blockage turns out to be in Sydney Water’s system, the plumber rings 13 20 90 to get a job number, and that number has to be included in any claim you make afterwards. It is worth asking whether they have done it.
Whose blockage is it, yours or Sydney Water's
Responsibility follows the pipe. You are responsible for the wastewater pipes on your property up to the point where they connect to Sydney Water’s network, and they are responsible for the network itself. Sydney Water owns the junction, while the property owner remains responsible for the connection into it, including tree roots that enter at that point.
Sydney Water’s own guidance on telling them apart:
- Likely theirs if the overflow is visible beyond your property boundary, your neighbours have the same problem, there is widespread flooding locally, or the overflow is continuous while you are not using water.
- Likely yours if your neighbours are fine, your drainage is not shared, there is no local flooding, and it only overflows when you use water on the property.
There is one useful exception worth knowing on a deep line. If excavation is needed to repair a fault at the point of connection and the pipe is more than 2.5 metres deep, Sydney Water takes over the repair, although they do not reimburse the cost of getting down to that depth.
What it costs to find and clear the blockage behind an overflowing gully
The gully is the symptom. The cost sits in finding and clearing whatever is blocking the line beneath it. The figures below are indicative ranges across the Sydney market, useful for judging a quote against, and not a quote themselves.
| Work | Indicative range | When it applies |
|---|---|---|
| Clearing a simple blockage | $150 to $400 | Reachable blockage, machine clears it on the day |
| Clearing a blocked main sewer line | $300 to $600 | The main line rather than a single fixture |
| High pressure jetting | $250 to $600 | Grease, sludge or a line that keeps closing over |
| Root cutting | $400 to $1,200 | Roots through the joints, common under the older canopy |
| CCTV drain inspection | $250 to $550 | Finding why it blocked, and whether the pipe is broken |
Clearing gets the water away. It does not tell you why the line closed over, which is why a CCTV inspection of the line is usually the more useful spend on a drain that has surcharged more than once. Where the camera finds a cracked or root damaged section, repairing the damaged section of sewer is what stops it recurring.
How to keep an overflow relief gully working
Most failed gullies were fine when they were installed. They stopped working because the ground around them changed, or because nobody looked at them for a decade.
- Keep the grate in place and clear. Leaves, mulch and soil over the top defeat it.
- Watch the ground level. New paving, turf or a garden bed that buries the riser removes the 75mm clearance the standard requires.
- Do not pave over it. If a path has to go there, the levels around it have to be graded so water cannot pond and get in.
- Pour a bucket of water down it occasionally if it smells. The trap holds a water seal, and a dry seal lets sewer gas up.
- Treat slow drains as the early warning. A gully only overflows once the line is properly blocked, and gurgling or slow emptying usually comes first.
If the same line keeps closing over, the pattern matters more than the individual clear, and it is worth reading why blocked drains keep coming back before paying for another visit. For anything urgent, clearing a blocked drain across the North Shore is what we do most days, and our North Shore plumbers work around the clock for a surcharge that will not wait.
Frequently asked questions
Can I cover or seal my overflow relief gully?
No. The grate is the release point, so sealing it does not prevent a surcharge, it redirects one into the house through the lowest fixture. The same applies to paving over it, putting a pot plant on it or weighing the lid down. If the grate is unsightly, the fix is landscaping around it that keeps the required clearance, not hiding it.
Why does my overflow relief gully smell?
Usually because the water seal in the trap has dried out, which lets sewer gas rise through the grate. It is most common in a spot that rarely gets used or in a dry spell. Pouring a bucket of water into the gully refills the seal. If the smell returns quickly or comes with slow drains inside, the line itself needs looking at.
Who pays if sewage overflows inside my house?
It depends where the blockage sits. You are responsible for the pipes on your property up to the point of connection, and Sydney Water is responsible for its network. If the blockage is in their line, call them on 13 20 90, and if a plumber attends first, they need to ring for a job number, because any claim has to include it.
Does every house have an overflow relief gully?
It is a standard requirement on connected properties, but older homes on the North Shore sometimes have one that no longer meets the height rules, or one that has been buried by later landscaping. If you cannot find a grate outside, or the one you have sits flush with the soil, that is worth having checked before the next wet week rather than after it.
In short
Keep the overflow relief gully accessible and uncovered. The normal clearances are at least 150mm below the lowest connected fixture and 75mm above the surrounding surface; paved or path areas have a specific exception where the surface prevents ponding and stormwater ingress. Have a licensed plumber assess the installation and any sewer overflow.
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Mathew is a licensed plumber, drainer and gasfitter, and the owner of Voyager Plumbing. He has held his own NSW contractor licence since December 2016 and is the nominated supervisor on the company licence.
Voyager Plumbing works across Sydney’s North Shore. NSW Contractor Licence 476657C, fully insured, available 24/7.
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