Overflow relief gully: what it is and why it's overflowing

What an overflow relief gully does

It's a short vertical pipe with a loose grate, usually outside near the bathroom or laundry. Under the Australian plumbing standard AS/NZS 3500.2, which NSW applies, its top must sit at least 150 mm below the lowest fixture inside, such as a shower grate or the water in the toilet bowl, and at least 75 mm above the ground around it. That height difference means a blocked sewer overflows outside first. The grate is meant to sit loose so it can lift off when sewage pushes up.

Why it's overflowing

• Tree roots in the joints of an old clay sewer pipe, the most common cause in established Sydney suburbs.
• Wipes, sanitary items or fat that have built up in the line.
• A cracked, collapsed or offset section of pipe that catches waste.
• A blockage in Sydney Water's sewer main, which usually affects neighbours too.
• Heavy rain getting into the sewer. A gully that only overflows in storms may not be a blockage on your side.

Is it your pipe or Sydney Water's?

• If your neighbours' gullies are overflowing too, or sewage is coming out of a maintenance hole in the street, call Sydney Water on 13 20 90. That's their main, and they clear it.
• If only your gully is overflowing, the blockage is almost always in your private sewer line, which the property owner pays to clear.
• Sydney Water owns the junction where your pipe joins its sewer, but the owner is responsible for the connection and for clearing tree roots that enter the junction at that point.

What to do right now

• Stop using water: no flushing, showers, dishwasher or washing machine. Every litre ends up at the gully.
• Keep children and pets away, and don't hose sewage into a stormwater drain.
• Leave the grate in place and uncovered. It's what stops sewage coming up inside.
• Book a licensed drainer who can jet the line from the gully and run a camera to find the cause. Block Drains clears a blocked sewer line for $549 fixed, any hour, with the camera inspection included.

Overflow relief gully vs a reflux valve

An overflow relief gully is the standard protection on a gravity sewer in Sydney. Where a gully can't sit low enough, for example in a basement bathroom, a plumber can fit a reflux (non-return) valve instead, which lets waste out but stops sewage flowing back in. A reflux valve needs checking and servicing; a gully doesn't. Homes connected to a pressure or vacuum sewer system, used in some parts of Sydney, work differently: Sydney Water owns that equipment, so call 13 20 90 if its alarm sounds.

Problems with the gully itself

• Buried or covered. Pavers, a garden bed or a new deck over the gully mean a blockage overflows inside instead. A plumber can raise or move it.
• Too high. After an extension or a new bathroom, the gully may no longer sit below the lowest fixture. A plumber can lower it, or fit a reflux valve where the height can't be met.
• Smelling. If the gully has a water trap that has dried out, sewer gas escapes. A bucket of water usually fixes it.

Questions

Should there be water in my overflow relief gully?

Many gullies hold a little water in a trap at the bottom, which is normal. Water rising up the pipe, or sewage spilling over the top, means the line is blocked.

Can I cover the overflow relief gully?

No. It has to stay uncovered and below the lowest fixture in the house. If it's covered, a sewer blockage overflows inside instead.

Who pays to clear an overflowing gully?

If the blockage is in your private sewer line, the property owner pays, which in most rentals means the landlord unless the tenant caused it. If Sydney Water confirms the blockage is in its main, it clears it at no cost to you.

Sources

• NSW Government: Overflow relief gully requirements
• Sydney Water: Responsibility for wastewater repairs
• Block Drains: how our Sydney price ranges are compiled