Digging a soakage trench for a greywater system yourself is potentially legal in New Zealand, but it's not as simple as just grabbing a spade. The earthworks themselves are generally something a homeowner can do, but the greywater system as a whole involves building consents, council approvals, and some plumbing connections that are restricted work requiring a licensed professional. Here's a clear breakdown of what you can and can't do.
What Is a Greywater Soakage System?
A greywater soakage system diverts wastewater from sources like showers, baths, and laundry (but not toilets or kitchen sinks, which produce blackwater) into a trench filled with aggregate, where it disperses into the soil. These systems are covered under the NZ Building Code, particularly clause G13 (Foul Water), which sets out how wastewater must be managed on-site.
Do You Need a Building Consent?
In most cases, yes — a building consent is required for a new greywater disposal system. Greywater systems are not typically listed under Schedule 1 of the Building Act 2004 as exempt work, meaning you'll almost certainly need to apply to your local council before starting.
Consent requirements can vary between councils, so always check with your local territorial authority before you begin. Some councils have specific local rules about setbacks from boundaries, waterways, and bores. You can find general guidance on the consent process at MBIE Building Performance.
Can You Legally Dig the Trench Yourself?
The physical earthworks — digging the trench — are not restricted work in themselves. A competent homeowner can legally dig a soakage trench on their own property, provided:
- You have the necessary building consent in place first
- The trench is designed and sized to meet your council's requirements and the NZ Building Code
- You comply with any conditions set out in your consent documents
- You call Before U Dig (0800 248 344) to locate any underground services before you start digging — this is essential for safety
For deeper trenches (generally over 1.5 metres), WorkSafe NZ has strict requirements around shoring and trench safety. Even at shallower depths, always be aware of the risk of trench collapse.
What Is Restricted (Licensed) Work?
This is where many homeowners get caught out. While you can dig the hole, connecting the greywater system to your home's existing plumbing is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006. This includes:
- Diverting or connecting drain lines from your shower, bath, or laundry
- Installing any pipework that forms part of the drainage system
- Certifying the completed drainage work
This work must be carried out by — or supervised and certified by — a licensed drainlayer or registered plumber. You can verify a tradesperson's licence at the Plumbers Gasfitters & Drainlayers Board. Doing restricted plumbing or drainlaying work yourself (or hiring an unlicensed person) is illegal and could void your home insurance.
Typical Soakage Trench Requirements
While your council and consent documents will specify exact requirements, soakage trenches for greywater systems commonly involve:
| Element | Typical Requirement |
|---|---|
| Trench depth | 450–600 mm (varies by soil type and council) |
| Aggregate fill | Clean graded gravel or crushed rock |
| Perforated pipe | Ag-pipe or similar, laid level or with slight fall |
| Geotextile fabric | Wrapped around aggregate to prevent silting |
| Setback from boundaries | Often 1.5–3 m minimum (check with council) |
| Setback from waterways | Varies — check with council and regional rules |
Always follow the specific design approved in your building consent — these are guides only.
Soil Testing and Site Suitability
Not all soils are suitable for soakage. Clay-heavy soils drain poorly and may mean a soakage trench won't work at all. Your council may require a percolation test (perc test) to assess your soil's absorption rate before granting consent. A geotechnical engineer or experienced drainlayer can carry this out. BRANZ has useful technical resources on on-site wastewater disposal if you want to understand the engineering principles.
Healthy Homes and Rental Properties
If the property is a rental, note that wastewater systems must comply with applicable tenancy and building requirements. Any work must be consented and compliant — cutting corners on a rental property carries additional legal risk. Any work must be consented and compliant — cutting corners on a rental property carries additional legal risk.
When to Call a Professional
You should engage a licensed drainlayer for:
- Designing the soakage system to meet Building Code clause G13
- All plumbing and drainage connections
- Certifying the completed work for your building consent sign-off
For the earthworks themselves, if the trench is deep, in difficult ground, or near services, hiring a professional earthmover or excavator operator will save time and reduce risk. You can find qualified local earthmovers and drainlayers on tradedirectory.co.nz.
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