Adding a new downpipe to an existing stormwater drain is something many NZ homeowners consider when extending a roof, adding a deck, or fixing drainage problems. The short answer is: some of the above-ground work you can do yourself, but connecting into the underground drainage system is restricted work that must be carried out by a licensed drainlayer. Getting this right matters — poor stormwater management can cause flooding, foundation damage, and even breach the NZ Building Code.
Understanding the Rules: Restricted Work and Licensing
In New Zealand, underground stormwater drainage work is classified as restricted building work and regulated drainlaying work under the Plumbers, Gasfitters, and Drainlayers Act 2006. This means any connection into an underground drain — including tapping into an existing stormwater line — must be done by, or under the supervision of, a licensed drainlayer.
You can check whether a tradesperson holds a current licence on the Plumbers Gasfitters & Drainlayers Board website before hiring them.
As a homeowner, you are generally permitted to:
- Install or replace the above-ground downpipe itself (the vertical pipe running down the outside of your house)
- Connect a downpipe to a surface-level channel or soakage pit that does not involve underground pipework
- Carry out minor maintenance like clearing blockages in accessible above-ground sections
You are not permitted to:
- Cut into or connect to an underground stormwater drain yourself
- Lay new underground drainage pipework
- Alter the underground drainage system in any way
Do You Need a Building Consent?
This is where things get a little more nuanced. Under Schedule 1 of the Building Act 2004, some minor drainage work is exempt from building consent, but the exemptions have specific conditions and are not blanket approvals.
As a general guide:
- Replacing a like-for-like downpipe in the same location is typically exempt
- Adding a new downpipe connection to an existing underground drain may require a building consent, depending on the scope of work and your local council's interpretation
Always check with your local council before starting work. Councils across NZ can interpret Schedule 1 exemptions differently, and getting it wrong can cause problems when you sell your home or make an insurance claim. The MBIE Building Performance website has detailed guidance on consent exemptions.
NZ Building Code Compliance
Any stormwater work must comply with NZ Building Code Clause E1 (Surface Water). This clause requires that stormwater is collected and disposed of in a way that does not cause damage to buildings or neighbouring properties. Your local council may also have specific requirements around:
- Stormwater discharge points — you generally cannot redirect stormwater onto a neighbour's property or into the sanitary sewer
- Soakage — in some areas, soakage pits or trenches are acceptable; in others (such as areas with high water tables or clay soils), they are not
- Impervious surface thresholds — adding large paved or roofed areas can trigger consent requirements related to increased stormwater runoff
For technical guidance on drainage design and best practice, BRANZ is an excellent NZ resource.
What the Job Typically Involves
Here is a general overview of what connecting a new downpipe to an existing stormwater drain looks like:
| Stage | Who Does It | Notes |
|---|---|---|
| Plan the route and check consent requirements | Homeowner + council | Do this first |
| Install or replace above-ground downpipe | Homeowner (DIY) | Use correct pipe size and brackets |
| Excavate and expose existing drain | Licensed drainlayer | Restricted work |
| Cut in and install a saddle or junction | Licensed drainlayer | Restricted work |
| Backfill and reinstate surface | Licensed drainlayer | Must be done correctly |
| Inspection (if consent required) | Council inspector | Book in advance |
Choosing the Right Pipe and Fittings
For above-ground downpipes, uPVC is the most common material used in NZ. Standard residential downpipes are typically 65 mm or 80 mm in diameter, though this depends on the roof catchment area. Your licensed drainlayer or a hardware store like Bunnings or Mitre 10 can help you select the right size. Underground stormwater pipes are generally 100 mm uPVC as a minimum.
NZ Climate Considerations
NZ's variable rainfall — particularly in regions like Northland, the West Coast, and parts of Auckland — means undersized stormwater systems can be quickly overwhelmed. If you are adding a downpipe because of a new roof area or extension, it is worth having a drainlayer assess whether your existing drain has sufficient capacity.
When to Call a Professional
You should always engage a licensed drainlayer for:
- Any connection into an underground stormwater drain
- New underground pipework or alterations to existing underground drainage
- Situations where you are unsure whether consent is required
- Diagnosing drainage problems such as slow-draining or blocked underground pipes
A qualified drainlayer will also be able to advise on whether your existing system can handle additional flow, and can handle the consent process on your behalf if required.
You can find licensed local drainlayers in your area right here on tradedirectory.co.nz.
Safety Note
Excavating around underground services carries real risk. Always contact Dial Before You Dig (0800 248 344) to identify the location of underground services — including power, gas, water, and telecoms — before any digging takes place. This is a legal requirement in many situations and a basic safety step in all of them.