Joining two different sized drain pipes together is technically possible using standard reducer fittings — but whether you're legally allowed to do it yourself in New Zealand depends heavily on where that pipe is located and what it connects to. For most underground or sanitary drainage work, this is restricted work that must be carried out by a licensed drainlayer.
What Makes Drain Work "Restricted" in NZ?
Under the Plumbers, Gasfitters, and Drainlayers Act 2006, drainlaying is a licensed trade. Restricted drainlaying work includes installing, altering, repairing, or connecting any part of a sanitary drain — that is, any pipe that carries wastewater or sewage from your home to the public sewer or an on-site wastewater system.
This means that if you're joining pipes that form part of your underground drainage system, or connecting to a soil stack (the vertical pipe carrying toilet waste), you must use a licensed drainlayer. Doing this work yourself — or hiring an unlicensed person — is illegal and could expose you to significant liability, particularly if it causes a blockage, leak, or public health issue.
You can check whether a drainlayer is licensed on the Plumbers Gasfitters & Drainlayers Board website.
What About Above-Ground or Minor Drainage Work?
There is a narrow category of work a homeowner may do themselves. Generally, you may:
- Clear a blockage in your own drain using a plunger or drain snake
- Replace a trap under a sink or basin (like-for-like, same size)
- Reconnect a waste fitting to an existing trap (e.g., after replacing a basin)
However, joining two different sized pipes — even above ground — typically involves altering the drainage system, which in most cases still falls under restricted work. If you're unsure, contact the PGDB or your local council before starting.
How Are Different Sized Pipes Actually Joined?
When a licensed drainlayer does need to join two pipes of different diameters, they use purpose-made fittings. Common NZ drainage pipe sizes include 40 mm, 50 mm, 65 mm, 100 mm, and 150 mm.
Common Reducer Fittings
| Fitting Type | Typical Use |
|---|---|
| Concentric reducer | Joins two pipes on the same centreline — common in horizontal runs |
| Eccentric reducer | Keeps the bottom of the pipe level — used to avoid air pockets |
| Rubber coupling reducer | Flexible join between two different pipe sizes or materials |
| Push-fit reducer | Used with PVC drainage systems for quick, solvent-free connections |
Most NZ residential drainage uses uPVC pipe conforming to AS/NZS 1260. Fittings must be compatible with the pipe material and size. Solvent-welded joints are standard for permanent underground connections; rubber-ring or push-fit joints are used where some movement is expected.
NZ Building Code Considerations
Drainage work must comply with the NZ Building Code, specifically Clause G13 (Foul Water), which sets out requirements for sanitary drainage systems including pipe sizing, gradients, and ventilation.
Pipe sizing isn't arbitrary — drains must be sized to handle the discharge units (flow load) from all connected fixtures. Joining a smaller pipe to a larger one in the wrong direction, or using an undersized pipe downstream, can cause backflow, slow drainage, or sewer gas issues. Getting this wrong can also mean your home fails a building inspection.
For most drainage alterations, a building consent may be required. Some minor work may be exempt under Schedule 1 of the Building Act, but you should always confirm with your local council before proceeding — exemptions are narrower than many homeowners assume.
Does the Healthy Homes Standard Apply?
If you're a landlord, note that the Healthy Homes Standards include drainage requirements. Faulty or non-compliant drainage can put you in breach of your obligations as a landlord, so it's especially important to use licensed tradespeople and ensure work is done to code.
When to Call a Professional
You should always engage a licensed drainlayer for:
- Any work on underground sanitary drains or stormwater connections
- Joining pipes of different sizes within a drainage system
- Connecting new fixtures (toilets, basins, showers) to existing drains
- Any work that alters the layout, gradient, or pipe sizing of your drainage
- Repairs to cracked, collapsed, or root-damaged drain pipes
A qualified drainlayer will ensure the work complies with the NZ Building Code, obtain any necessary consents, and issue the required Certificate of Compliance. This protects you legally and is important for insurance and future property sales.
You can find licensed local drainlayers on tradedirectory.co.nz — always check their PGDB licence before work begins.
Certifying Drainlayer
