A slightly collapsed underground drain is more serious than a standard blockage — the pipe itself has been damaged or deformed, which means simply plunging or rodding may not fix the problem and could make things worse. Before you reach for the drain snake, it pays to understand what you're dealing with and what your options are as a NZ homeowner.
What Does a "Slightly Collapsed" Drain Actually Mean?
A partial collapse (sometimes called a pipe deformation or pipe sag) means the drain pipe underground has cracked, shifted, or partially caved in — but hasn't completely failed. Common causes in New Zealand include:
- Tree root intrusion cracking older clay or concrete pipes
- Ground movement from soil settlement, earthquakes, or heavy vehicles driving over the pipe
- Age and corrosion in older clay or cast-iron pipes
- Poorly compacted backfill when the pipe was originally laid
- High water table or soil washout undermining the pipe
Signs that your drain may have partially collapsed (rather than just being blocked) include slow drainage that keeps returning after clearing, gurgling sounds, wet patches or sinkholes in the yard above the drain line, and sewage smells.
Step 1 — Confirm the Problem With a CCTV Drain Inspection
Before doing anything else, the most important step is to get a CCTV drain camera inspection. This is the only reliable way to confirm whether the pipe has actually collapsed, where the damage is, and how severe it is. Many licensed drainlayers offer this service, and it will save you money in the long run by avoiding guesswork.
A camera inspection will show:
- The exact location and depth of the damage
- Whether the collapse is minor (a crack or slight deformation) or major
- Whether tree roots are involved
- The condition of the rest of the pipe
Step 2 — Understand What You Can and Cannot Do Yourself
In New Zealand, drainlaying is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006. This means that any work on underground drains — including excavating and repairing or replacing a collapsed section — must be carried out by, or under the supervision of, a licensed drainlayer. You can check licensing at the Plumbers Gasfitters & Drainlayers Board.
What a homeowner CAN do:
- Clear a simple surface blockage at an inspection point (e.g., lifting a gully trap lid and removing debris by hand)
- Use a plunger on an internal fixture
- Identify symptoms and gather information for the tradesperson
- Arrange a CCTV inspection
What requires a licensed drainlayer:
- Any excavation and repair of underground drain pipes
- Pipe relining or pipe bursting (trenchless repair methods)
- Replacing a collapsed section of drain
- Any connection to the public sewer or stormwater network
Step 3 — Know Your Repair Options
Once the CCTV inspection is done, a licensed drainlayer will recommend one of the following approaches depending on the severity:
| Repair Method | Best For | Disruption Level |
|---|---|---|
| High-pressure water jetting | Minor sags with partial blockage, root clearing | Low |
| Pipe relining (CIPP) | Cracked or slightly deformed pipes with intact shape | Low — no digging |
| Pipe bursting | Replacing pipe without full excavation | Medium |
| Open excavation & replacement | Severely collapsed or badly misaligned sections | High |
Pipe relining (Cured-In-Place Pipe lining, or CIPP) is increasingly popular in NZ because it avoids digging up driveways or gardens. A flexible liner is inserted and cured in place, creating a new pipe within the old one. It's worth asking your drainlayer whether your pipe is a suitable candidate.
Step 4 — Building Consent Considerations
Minor drain repairs (like-for-like replacement of a short section) may be exempt from building consent under Schedule 1 of the Building Act 2004, but this depends on the scope of work and your local council's interpretation. Always check with your local council before work begins — don't assume it's exempt. Your licensed drainlayer should be familiar with local requirements and can advise you. For general guidance, see MBIE Building Performance.
Step 5 — Temporary Measures While You Wait
If the drain is still partially functional and you're waiting for a tradesperson:
- Reduce water use through the affected drain where possible
- Do not use chemical drain cleaners — these won't fix a structural problem and can damage pipes further
- Avoid driving vehicles over the area above the collapsed pipe
- If sewage is backing up into the house, this is an urgent situation — call a drainlayer immediately
When to Call a Professional
You should call a licensed drainlayer as soon as you suspect a collapsed drain. This is not a DIY repair — underground drain work is legally restricted in NZ, and attempting it yourself could result in fines, failed inspections, or making the damage significantly worse. If sewage is surfacing in your yard or backing up inside, treat it as an emergency and call immediately. You can find qualified local drain cleaners and drainlayers on tradedirectory.co.nz.
For guidance on what's considered healthy and safe drainage in rental properties, the Tenancy Services Healthy Homes standards may also be relevant.
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