Reading a site plan before any excavation is one of the most important steps you can take to protect yourself, your property, and the people around you. In New Zealand, striking a buried gas line, electrical cable, or water main can be life-threatening and extremely costly — so understanding what's on paper before the bucket hits the ground is non-negotiable.
What Is a Site Plan?
A site plan (sometimes called a services plan or engineering drawing) is a scaled, bird's-eye-view drawing of a property showing the layout of structures, boundaries, and — critically — the location of underground and overhead services. These may include:
- Electrical cables (low voltage and high voltage)
- Gas mains and service lines
- Water supply pipes
- Stormwater and wastewater drains
- Telecommunications and fibre-optic cables
- Irrigation lines
Site plans are typically provided by your local council, a licensed surveyor, or the project engineer. If you're working on a building project, the plans should be part of your building consent documentation. Always confirm with your local council what plans are available for your site.
Understanding Scale and Orientation
Every site plan has a scale ratio — for example, 1:200 means 1 mm on the plan equals 200 mm (or 0.2 m) on the ground. Use a scale ruler or the plan's own scale bar to measure distances accurately. Never estimate by eye.
Check the north arrow so you can orient the plan to match the actual site. Stand at a known reference point (like the front boundary or a corner of the house) and rotate the plan to align with what you see in front of you.
Reading the Legend and Symbols
Every plan should include a legend (key) explaining what each line, colour, and symbol means. Common conventions in NZ include:
| Service | Common Colour/Line Style |
|---|---|
| Electrical cables | Red dashed line |
| Gas pipes | Yellow line |
| Water supply | Blue line |
| Stormwater | Light blue or green |
| Wastewater/sewer | Brown or dark green |
| Telecommunications | Orange or purple |
These colours are not universal — always refer to the specific legend on your plan rather than assuming.
Locating Buried Services on Site
A site plan gives you a starting point, but plans are not always accurate. Pipes and cables may have been laid off-line, depths may vary, and older services may not be recorded at all. Here's how to verify:
1. Contact Before You Dig
In New Zealand, contact Dial Before You Dig (now operating through various utility companies and councils). Many local authorities and network operators can provide updated service location information. Contact your local council and each relevant utility (electricity, gas, water, telecommunications) directly to request up-to-date as-built drawings.
2. Use a Cable and Pipe Locator
A cable avoidance tool (CAT) or electromagnetic locator can detect buried metallic services. These can be hired from equipment hire companies. Note that non-metallic pipes (like some PVC water lines) may not be detectable this way.
3. Hand-Dig Trial Holes
Within 600 mm either side of any marked service, always hand-dig carefully rather than using machinery. This is standard safe excavation practice and is strongly recommended by WorkSafe NZ.
Understanding Depths and Clearances
Buried service depths vary. As a general guide (always verify with the relevant utility):
- Electrical cables: typically 600 mm deep in open ground, but can be shallower near structures
- Gas pipes: typically 450–600 mm deep
- Water mains: typically 450–750 mm deep
- Telecommunications: often 300–450 mm deep
Because depths can vary significantly — especially in older properties or where ground levels have changed — never rely on depth alone. Always treat any unmarked area as potentially containing services.
NZ Building Code and Consent Considerations
If your excavation is part of a building project, check whether a building consent is required. Under Schedule 1 of the Building Act, some minor excavation work may be exempt, but this depends on the scope and proximity to structures. Always confirm with your local council before starting — requirements vary by region.
For excavation near boundaries or retaining walls, MBIE Building Performance provides guidance on what triggers consent requirements. Excavation affecting neighbouring properties or public land has additional legal obligations.
Safety on Site
Never assume an area is clear. Even with a site plan in hand, treat every dig as potentially hazardous until services are physically confirmed. Key safety rules:
- Mark all known services on the ground with spray paint or flags before excavating
- Maintain a safe exclusion zone around confirmed service locations
- If you strike an unidentified pipe or cable, stop immediately, move people away, and contact the relevant utility
- If you hit a gas line, evacuate the area and call 111 and your gas network operator immediately
- Wear appropriate PPE including high-visibility clothing and safety boots
For full guidance on excavation safety, refer to WorkSafe NZ.
When to Call a Professional
Reading a site plan and planning a safe dig is a skill — and for anything beyond simple, shallow garden work, it pays to bring in a qualified excavator operator or civil contractor. A professional will:
- Have experience interpreting as-built drawings and engineering plans
- Know how to use service detection equipment correctly
- Understand their legal obligations under the Health and Safety at Work Act 2015
- Carry appropriate insurance
Any work involving electrical, gas, plumbing, or drainlaying connections must be carried out by a licensed professional. Electrical work is restricted under the Electrical Workers Registration Board (EWRB), and plumbing and gasfitting is regulated by the Plumbers, Gasfitters and Drainlayers Board (PGDB). Even if you're doing the excavation yourself, the connection or repair of any service must be done by the appropriate licensed tradesperson.
You can find qualified, local excavation contractors and civil trades on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
