For most DIY excavation work in New Zealand, 1.5 metres is the critical depth threshold. Beyond this point, the risks to personal safety increase dramatically, specific legal obligations kick in, and you'll almost certainly need professional help. Even at shallower depths, there are important rules around building consents, underground services, and soil stability that every homeowner should understand before picking up a spade or hiring a mini digger.
The 1.5 Metre Rule Explained
Under the NZ Building Code and general safe excavation practice, any excavation deeper than 1.5 metres is considered a significant hazard. At this depth, unsupported trench walls can collapse without warning — even in soil that looks stable. A cubic metre of soil can weigh over a tonne, and trench collapses are a leading cause of serious injury and death on worksites.
WorkSafe NZ sets out clear requirements for managing excavation hazards. For any excavation deeper than 1.5 m, you must have a competent person assess the ground conditions and implement appropriate controls — such as shoring, benching, or battering the sides. For most homeowners, this means engaging a professional excavator or civil contractor.
Do You Need a Building Consent?
This is where things get a bit more nuanced. Under the Building Act 2004, some excavation work is exempt from building consent under Schedule 1, but the exemptions have conditions. As a general guide:
- Shallow landscaping and garden work (typically less than 1.5 m deep and not affecting foundations or drainage) is usually exempt.
- Excavation near existing foundations — even at shallow depths — may require consent, as it can undermine structural support.
- Retaining walls over 1.5 m in height (or in certain ground conditions) generally require a building consent.
- Earthworks that could affect stormwater or neighbouring properties may trigger resource consent requirements under your local district plan.
Always check with your local council before you start. Exemptions vary by council and site conditions, and getting it wrong can be costly. You can find general guidance at MBIE Building Performance.
Underground Services — Know Before You Dig
Before any excavation, you must locate underground services. Striking a gas line, water main, or power cable can be fatal and is your legal liability.
- Contact Dial Before You Dig (now called beforeUdig in NZ) to request service location information from network operators.
- Your local council can often provide drainage and water main plans.
- Consider hiring a cable and pipe locator — many hire companies stock these, or a professional can do a sweep.
Never assume services aren't present. Even in older suburbs, unmarked pipes and cables are common.
Soil Types and Stability
Not all soil behaves the same way. NZ has a wide range of ground conditions — from stable clay in parts of Canterbury to loose volcanic soils in Auckland and Wellington's notoriously variable terrain.
| Soil Type | Risk Level | Notes |
|---|---|---|
| Firm clay | Lower | Can still collapse if disturbed or wet |
| Sandy or gravelly soil | High | Collapses quickly, little warning |
| Fill or made ground | Very high | Unpredictable, common in NZ subdivisions |
| Volcanic/pumice soils | High | Loose and unstable when excavated |
If you're unsure of your soil type, a geotechnical engineer or experienced excavation contractor can assess the site before work begins.
What Can a Homeowner Do Themselves?
For shallow excavations under 1.5 m that don't affect foundations, drainage, or neighbouring properties, a capable homeowner can generally:
- Dig garden beds, planting holes, and shallow trenches for irrigation.
- Excavate for small concrete pads (check consent requirements first).
- Use a hired mini excavator for larger shallow earthworks — many hire companies offer these with a brief operator induction.
Always work with a buddy present, never enter a trench alone, and keep spoil (dug soil) well back from the edge — at least 0.6 m — to avoid it falling back in.
When to Call a Professional
You should engage a qualified excavation contractor when:
- The excavation will exceed 1.5 metres in depth.
- You're working near existing building foundations, retaining walls, or boundary structures.
- The ground is unstable, waterlogged, or made-up fill.
- You need to install or modify drainage, stormwater, or sewer connections — this is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out (or supervised) by a licensed drainlayer registered with the Plumbers Gasfitters & Drainlayers Board.
- A building consent is required for the associated work.
- The project involves significant earthworks that could affect neighbouring properties or waterways.
A professional excavator will have the right equipment, insurance, and knowledge of local ground conditions. You can find qualified local excavation contractors on tradedirectory.co.nz.
A Note on Health and Safety
WorkSafe NZ treats excavation as a high-risk activity. Even as a homeowner doing DIY work, you have a duty of care to anyone on or near your site. If you're hiring contractors, you also take on responsibilities as a Person Conducting a Business or Undertaking (PCBU) under the Health and Safety at Work Act 2015. When in doubt, get professional advice.
Licensed Building Practitioner (LBP)
