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Water on a North Shore block, a guide by Dotto

Plumbing and drainage on Sydney’s North Shore, New South Wales

The six guides

Guide · Drainage

The overflow relief gully

An overflow relief gully must sit at two set heights: its top at least 150 mm below the lowest fixture connected to the drain, and at least 75 mm above the finished ground around it, except in a path or a paved area, where it is finished at a level that prevents ponding and water getting in. Building Commission NSW says that missing those clearances can cause significant property damage if the sewer surcharges or blocks.

Section through a house wall. Inside, a floor waste gully at floor level. Outside, the overflow relief gully riser rises out of the ground. Its top is marked at least 150 millimetres below the floor waste grate, and at least 75 millimetres above the finished ground. FLOOR INSIDE1 OUTSIDE WALL3 FLOOR WASTE GULLY2 FINISHED GROUND5 OVERFLOW RELIEF GULLY4 150 mm6 AT LEAST 75 mm7 AT LEAST
  1. 1Floor inside the house
  2. 2Floor waste gully, the lowest fixture in this drawing
  3. 3Outside wall
  4. 4Overflow relief gully
  5. 5Finished ground
  6. 6At least 150 mm from the top of the riser up to the lowest fixture, here the floor waste grate
  7. 7At least 75 mm from the finished ground up to the top of the riser
Drawn for this guide from the two clauses quoted below. Not to scale: the gaps are shown larger than life so they can be read.

Where the rule comes from

In June 2026 Building Commission NSW issued an advisory note, Note 1/2026, on overflow relief gullies. It applies to all of NSW and was written in response to non-compliant gullies found in the commission’s inspections: an ongoing issue with gullies installed without the required height clearances. It explains the requirements of the Australian Standard AS/NZS 3500.2:2026 for plumbers, builders and building designers.

The first height: below the lowest fixture

Clause 4.6.6.6 of the standard says the top of the overflow gully riser must be at least 150 mm lower than the lowest fixture connected to the drain. Because the gully’s top must sit below the lowest fixture, it ends up lower than every fixture on that drain. Where the 150 mm is measured from depends on the fixture:

Points of measurement, from Table 4.6.6.6 as quoted in Building Commission NSW’s note.
FixtureMeasured from
A soil fixture with an integral trapThe top surface level of the water seal
A floor waste gully or a showerThe top surface level of the grate
A soil fixture in an outbuilding or room whose floor is graded to an outside doorwayThe overflow rim of the fixture
Other fixtures, including greywater diversion devicesThe top surface level of the fixture outlet

The height is a vertical measurement, taken from the gully riser’s overflow level or from the overflow pipe’s invert.

The second height: above the ground

Clause 4.6.6.7 requires the top of the gully riser (or the overflow pipe’s invert) to sit at least 75 mm above the finished surface level around it. There is one exception. Where the gully riser is in a path or a paved area, it “shall be finished at a level so as to prevent the ponding and ingress of water”.

Gardens and paths change the ground

The note asks plumbers to put the gully where both heights can be achieved, and to consider any future landscaping or paths that could change them. It is one reason to know where the gully is before you raise a garden bed or lay a path beside the house.

Whose job it is to get right

  1. The plumber sets the levelsLicensed plumbers are responsible for working out the correct gully levels before they start, which means checking final slab heights and finished ground levels with the builder.
  2. The builder and designer allow for itBuilders and building designers must make sure the slab and ground levels let the plumbing comply, including the gully, as the National Construction Code’s drainage provisions require.

When the heights cannot be met

Where a plumber cannot achieve the 150 mm height below the lowest fixture, clause 4.5.2 of the standard says a reflux valve shall be installed. The NSW variation to the Plumbing Code of Australia then requires that no sanitary drainage or sanitary plumbing waste stack is installed upstream of that valve.

Three more points from Building Commission NSW’s reflux valve note:

  • “Where a surcharge is likely to occur a reflux valve shall be installed immediately downstream of the boundary trap or inspection shaft.” The invert of its outlet must sit at least 80 mm higher than the invert of the water utility’s system it connects to.
  • Where a reflux valve is installed, it remains the property owner’s responsibility.
  • Network utility operators, the note names Sydney Water among them, may set their own soffit height requirements and may also know of surcharge conditions in their mains. “Where such conditions exist, a reflux valve must be installed in accordance with NSW Variation C2D4(d).”

The size of a gully, other places it may go, rules for flood-affected areas, and when one may be left out are all in clause 4.6.6 of the standard, which the note points to rather than repeats.

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