Can I Use a Digger to Bury a Large Water Storage Tank Myself?

Burying a large water storage tank with a digger is something many NZ homeowners tackle themselves — especially on rural properties where rainwater collection is essential. However, before you fire up the excavator, there are important rules around building consent, restricted plumbing work, and safe excavation practices that you need to understand. Getting these right from the start will save you time, money, and potential legal headaches.

Do You Need a Building Consent?

This is the first question to answer. Under the NZ Building Code, burying a large tank may or may not require a building consent depending on its size, use, and your local council's rules.

Schedule 1 of the Building Act lists certain exempt building work — some small tanks and cisterns may qualify, but larger buried tanks (particularly those connected to a potable water supply) often fall outside the exemption. Key factors include:

• Tank capacity — larger tanks (thresholds vary by council and tank type — check with your local council) are more likely to require consent.
• Intended use — tanks supplying drinking water are subject to stricter scrutiny.
• Proximity to boundaries, buildings, or drainage fields — setback requirements apply.
• Soil conditions and depth — deep excavations may trigger additional requirements.

Always confirm with your local council before starting work. Do not assume your project is exempt — councils interpret Schedule 1 differently, and getting it wrong can mean costly remediation or issues when you sell your property.

Choosing the Right Tank for Burial

Not all water tanks are designed to be buried. Above-ground polyethylene tanks are not rated for underground installation — the soil pressure will crush them. You need a tank specifically engineered and rated for burial, such as:

• Fibreglass (GRP) tanks — widely used in NZ for underground water storage.
• Concrete tanks — very durable but extremely heavy; specialist equipment is usually required.
• Rated polyethylene tanks — some manufacturers produce underground-rated poly tanks; always check the manufacturer's specifications.

Check with your tank supplier that the product is certified for underground use and follow their installation manual precisely — including bedding material, backfill requirements, and any internal bracing needed during installation.

Planning the Excavation

Once you have confirmed consent requirements and chosen your tank, careful excavation planning is critical.

Locate Underground Services First

Before any digging, you must identify all underground services. Contact Dial Before You Dig (now called Beforeudig in NZ) or check with your local council and network utility operators. Striking a buried power cable, gas line, or water main is extremely dangerous and potentially fatal.

Excavation Depth and Dimensions

• Allow adequate clearance around the tank for bedding and backfill — typically at least 300 mm on all sides, but follow the tank manufacturer's guidelines.
• The base of the excavation must be level and firm. A compacted sand or pea gravel bed is commonly used to support the tank evenly and prevent point loading.
• Trench safety is critical. Under WorkSafe NZ guidelines, excavations deeper than 1.5 metres require shoring, benching, or battering of the sides to prevent collapse. Even shallower excavations can be dangerous in loose or wet soils — never enter an unshored trench.

Digger Size and Access

For most residential buried tank installations, a 3–5 tonne excavator is typically sufficient, though larger concrete tanks may require a bigger machine. Consider:

• Site access for the digger and the tank delivery vehicle.
• Ground conditions — wet or soft ground may require tracked machines or ground protection matting.
• Spoil management — where will the excavated soil go? You may need a skip bin or space on-site.

Backfilling and Compaction

How you backfill around the tank is just as important as the excavation. Poor backfill can cause the tank to shift, deform, or fail over time.

Avoid using heavy mechanical compactors directly against the tank walls — this can cause deformation. Always follow the tank manufacturer's backfill specification.

Plumbing Connections — Restricted Work

Connecting your buried tank to any household water supply, pump system, or reticulation is restricted plumbing work under the Plumbers, Gasfitters and Drainlayers Board (PGDB). This work must be carried out by a licensed plumber or drainlayer. Attempting restricted work yourself is illegal and can void your insurance.

As a homeowner, you may generally:

• Dig the hole and prepare the site.
• Place the tank (with appropriate equipment and help).
• Do basic landscaping and reinstatement.

You must use a licensed tradesperson for:

• Any connection to potable (drinking) water supply lines.
• Overflow and drainage connections.
• Pump installations connected to the water system.

When to Call a Professional

Consider engaging a qualified excavator operator or civil contractor if:

• The excavation is deep (over 1.5 m), in unstable soil, or near existing structures.
• You are unfamiliar with operating an excavator safely.
• The tank is large or heavy and requires precise placement.
• Site access is difficult or ground conditions are poor.

A licensed plumber must handle all pipe connections regardless. For the full installation, a combination of a professional excavator operator and a licensed plumber will ensure the job is done safely and to code. You can find qualified local tradespeople at tradedirectory.co.nz.

Safety Reminder

Excavation is one of the most hazardous activities on a construction site. Trench collapse can be fatal with little or no warning. Always follow WorkSafe NZ safe excavation guidelines, never work alone in or near an open excavation, and ensure the area is fenced off from children and bystanders.

Sources & Further Reading

• MBIE Building Performance — Building Consent Exemptions
• WorkSafe NZ — Excavation Safety
• Plumbers, Gasfitters and Drainlayers Board — Restricted Work
• BRANZ — Build Magazine Technical Guidance
• Tenancy Services — Healthy Homes Standards