NZ Drain Grade Standards — AS/NZS 3500.3
Minimum drain grades in NZ are specified in AS/NZS 3500.3 Plumbing and Drainage — Stormwater Drainage, which forms part of the NZ Building Code under clause E1 (Surface Water) and E2 (External Moisture).
Minimum Grades for Foul Drains (Wastewater)
| Pipe size | uPVC minimum grade | Concrete/Clay minimum |
|---|---|---|
| 100mm | 1:50 | 1:55 |
| 150mm | 1:70 | 1:80 |
| 225mm | 1:100 | 1:100 |
| 300mm+ | 1:150 | 1:150 |
Minimum Grades for Stormwater Drains
Stormwater drains can generally be flatter than foul drains:
| Pipe size | Minimum grade |
|---|---|
| 100mm | 1:100 |
| 150mm | 1:150 |
| 225mm | 1:200 |
| 300mm+ | 1:300 |
Self-Cleaning Velocity
The minimum grades above are set to achieve self-cleaning velocity — typically 0.7 m/s for foul drains. At this flow rate, solids are carried along rather than settling. Drains flatter than the minimum will block repeatedly.
When Engineering Is Required
Drains that cannot achieve minimum grades due to site constraints may require engineering solutions — pump stations, vacuum drainage, or pressure systems. Always flag this to the client before starting work, as it significantly affects cost.
All drainage work must be performed by a registered drainlayer and inspected by the local council before backfilling. Self-certification is being expanded but as of 2026, council inspection is still required for most buried drainage.