Overflow relief gully requirements
This advisory note identifies common non-compliances related to overflow relief gully (ORG) installations across NSW. It provides practical guidance for licensed plumbers, contractors, builders and building designers on meeting installation requirements.
| Note | 1/2026 |
| Date issued | June 2026 |
| Last updated | June 2026 |
| For | Licensees and contractors working in the plumbing and construction |
| Issued by | Building Commission NSW, Practitioner Compliance, Plumbing Team |
| Scope | Applies to all of NSW |
Purpose
This advisory note has been developed in response to non-compliant overflow relief gully (ORG) installations identified during Building Commission NSW inspections.
It supports licensed plumbers, contractors, builders and building designers to understand the requirements for installing ORGs in NSW in accordance with AS/NZS 3500.2:2026.
Issue
Building Commission NSW has identified an ongoing issue with ORGs being installed without meeting the required height clearances. Failure to achieve these clearances can result in significant property damage in the event of a sewer surcharge or blockage.
Licensed plumbers are responsible for determining the correct ORG levels before commencing work. This requires coordination with the builder to confirm final slab heights and finished ground levels.
Builders and building designers must also ensure that the relationship between slab levels and ground levels is appropriately considered so the installation can comply with plumbing requirements, including those relating to ORGs, as outlined in the National Construction Code (NCC) Volume Two, Part 3.1.3 Drainage.
References
- AS/NZS 3500.2:2026
- NCC (BCA) Volume Two, Housing Provisions for site preparation
- NCC Volume Three - Plumbing Code of Australia
- Reflex vales requirements advisory note
Clause requirements
AS/NZS 3500.2:2026
Clause 4.6.6.6 - Height of overflow point below lowest fixture
A minimum height of 150 mm shall be maintained between the top of the overflow gully riser and the lowest fixture connected to the drain.
This height shall be measured vertically from the overflow level of the gully riser or from the invert level of the overflow pipe to the point given in Table 4.6.6.6.
Table 4.6.6.6 - Point of measurement on fixtures for heigh above overflow gully
| Fixture | Point of measurement |
|---|---|
| Soil fixture with an integral trap | Top surface level of the water seal |
| Floor waste gully or shower | Top surface level of the grate |
| Soil fixture located in an outbuilding or room, the flow of which is graded to an external doorway | Overflow rim of the fixture |
| Other fixtures (including greywater diversion devices) | Top surface level of the fixture outlet |
Clause 4.6.6.7 - Height above surrounding ground
The minimum height between the top of the overflow gully riser, or the invert of the overflow pipe, and the finished surface level shall be 75 mm, except where the gully riser is located in a path or a paved area, where it shall be finished at a level so as to prevent the ponding and ingress of water.
Regulatory commentary
Industry practitioners should take care to ensure that the ORG is installed in an appropriate location on the property where minimum height requirements can be achieved. Consideration must also be given to any future landscaping or pathways that may be constructed which may impact the relevant heights.
See examples of compliant ORG scenarios.
Example 1
Example 2
When height requirements cannot be met
Where practitioners are unable to achieve ORG height requirements, a reflux valve may be installed in accordance with Clause 4.5.2, however consideration must be made to the requirements of the Plumbing Code of Australia NSW Variation C2D4 (c), which prescribes that a sanitary drainage or sanitary plumbing waste stack is not installed upstream of the reflux valve.
See typical example below of an installation where a reflux valve has been installed to protect the ground floor fixtures, with a sanitary plumbing waste stack carefully installed to not pass through the reflux valve.
For more information about installation requirements, refer to the Reflux valves requirements advisory note.
Information contained within this advisory note refers to codes and standards current at the time at when it was last updated.
Building Commission NSW disclaims any liability (including for negligence) to any person in respect of anything done, or not done, by that person in whole or partial reliance on any of the information in this advisory note.