In many situations, yes — a NZ homeowner can dig a trench for drainage pipes themselves. However, the physical digging is only part of the job. Who connects, lays, and certifies the drainage pipes is where NZ law draws a firm line, and getting this wrong can be costly and even dangerous.
What the Law Says About Drainlaying in New Zealand
Under the Plumbers, Gasfitters and Drainlayers Act 2006, drainlaying is restricted work in New Zealand. This means that connecting, laying, altering, or repairing any drain that carries sewage or stormwater to a public system must be carried out (or supervised) by a Licensed Drainlayer. You can check licensing requirements and find registered practitioners through the Plumbers Gasfitters & Drainlayers Board.
As a homeowner, you are generally permitted to:
- Dig the trench itself (by hand or with hired machinery)
- Backfill the trench once the licensed work has been inspected and approved
- Do basic landscaping around the drainage area
You are not permitted to connect pipes to the public sewer or stormwater network, or carry out any restricted drainlaying work, unless you hold the appropriate licence.
Do You Need a Building Consent?
This depends on the scope of work. Minor drainage work on private property may fall under Schedule 1 exemptions of the Building Act 2004, but this is not always straightforward. For example, installing a new subsoil drain or altering an existing drainage system may require a building consent. Always check with your local council before starting work — requirements vary by region and by the nature of the drainage involved. The MBIE Building Performance website has useful guidance on when consents are required.
Planning Your Trench
Before you pick up a spade or hire an excavator, good planning will save you time, money, and headaches.
Check for Underground Services
This is critical. Striking a buried power cable, gas line, or water main can be life-threatening. Before digging:
- Contact Dial Before You Dig (0800 248 344) or visit their website to request a service location
- Ask your local council for any drainage plans for your property
- Use a cable/pipe locator (available for hire) if you're unsure
WorkSafe NZ has clear guidance on excavation safety that every DIYer should read before starting.
Trench Depth and Gradient
Drainage pipes need to be laid at the correct fall (gradient) to work properly — typically a minimum of 1:60 for stormwater and 1:40 for foul drainage, though your licensed drainlayer or council can confirm the exact requirements for your situation. Pipes also need to be buried deep enough to avoid damage and frost (less of a concern in most of NZ, but relevant in colder inland areas).
Trench Width and Bedding
- Trench width should allow comfortable pipe laying and compaction of bedding material on each side
- Most drainage pipes require a granular bedding layer (such as pea gravel or AP20 chip) beneath and around the pipe to support it and allow water movement
- Check the pipe manufacturer's specifications for bedding requirements
Hiring Excavation Equipment
For longer or deeper trenches, hiring a mini excavator is far more practical than hand digging. Mini excavators (1.5–3 tonne) are widely available from hire companies and are manageable for most homeowners with a bit of care. Larger machines may require an operator's ticket depending on the hire company's requirements.
You can find earthmoving and excavation professionals on tradedirectory.co.nz if you'd prefer to have the digging done for you.
Trench Safety
⚠️ Safety first: Trench collapses are a leading cause of serious injury and death on worksites. Any trench deeper than 1.5 metres must be shored, battered back, or otherwise made safe in accordance with WorkSafe NZ guidelines. Even shallower trenches can collapse in loose or wet soil. Never enter an unsupported trench.
Key safety points:
- Keep excavated soil well away from the trench edge (at least 600 mm)
- Never work alone in or near a deep trench
- Fence off the trench if leaving it unattended, especially around children
- Be aware of ground conditions — wet or sandy soils are particularly unstable
Stormwater vs. Subsoil vs. Sewer Drainage
Not all drainage is the same, and the rules differ:
| Drainage Type | Typical DIY Digging? | Licensed Drainlayer Required? |
|---|---|---|
| Subsoil/agricultural drain (private, no connection to public system) | Usually yes | Often not, but check with council |
| Stormwater drain (connecting to public network) | Digging yes | Yes, for connection |
| Foul/sewer drain | Digging yes | Yes, always |
For guidance on how drainage relates to the NZ Building Code (particularly Clause G13 – Foul Water and Clause E1 – Surface Water), visit MBIE Building Performance.
When to Call a Professional
You should engage a Licensed Drainlayer for any work involving connections to the public sewer or stormwater network, for complex drainage layouts, or if you're unsure about gradients, pipe sizing, or compliance. If the trench is deep, in unstable ground, or near existing services, hiring a professional earthmover or excavation contractor is the safest and most efficient option. Find qualified local tradespeople at tradedirectory.co.nz.
Site Safe
