Digging around an old buried oil tank is not a straightforward DIY job — it involves real risks including soil contamination, fire hazard, and potential legal obligations under NZ environmental law. While a homeowner can do some preparation work, the actual removal of a buried oil tank almost always requires qualified professionals and, in many cases, council notification or consent.
Understand What You're Dealing With First
Buried oil tanks — often old diesel or heating oil tanks — can leak over time, contaminating the surrounding soil and potentially groundwater. Before you pick up a spade, you need to know:
- What the tank held (heating oil, diesel, petrol, or another fuel)
- Whether it has leaked — look for staining, dead vegetation, or a petroleum smell in the area
- The tank's size and material — older tanks are often steel and may be corroded
- Your local council's requirements — some councils require notification or a resource consent before removal, especially if contamination is suspected
Contact your local council early. Under the Resource Management Act (RMA), disturbing contaminated land can trigger legal obligations, and your council's environmental health team can advise you on what's required in your area.
Check for Underground Services First
Before any digging, always call Dial Before You Dig (0800 248 344 or beforeudig.co.nz) to identify buried pipes, cables, and other services. Striking a gas line or electrical cable during excavation is extremely dangerous.
WorkSafe NZ has clear guidance on excavation safety, including requirements for managing the risks of trench collapse — check current WorkSafe NZ guidance for applicable depth thresholds and shoring requirements. If your tank is deep, you may need temporary shoring to prevent trench collapse.
Assess Contamination Risk Before Digging
If there's any sign of a leak — petroleum smell, oily sheen on nearby water, or discoloured soil — stop and get a site assessment done by an environmental consultant before proceeding. Disturbing contaminated soil without proper controls can spread contamination further and may expose you to liability.
The MBIE Building Performance website and your regional council can point you toward the correct process. In some cases, the site may need to be listed on the Hazardous Activities and Industries List (HAIL) register, which triggers specific requirements under your regional council's guidelines.
Safe Excavation Steps (Where DIY Is Appropriate)
If a professional assessment confirms there is no significant contamination and your council has no objections, here is how to approach the excavation safely:
1. Drain and Degass the Tank First
- Never excavate around a tank that still contains fuel. The tank must be pumped out and degassed by a qualified fuel technician before any digging begins.
- Residual vapours in an empty tank can still be explosive. A technician will use gas-detection equipment to confirm the tank is safe.
2. Mark Out the Dig Area
- Identify the tank's approximate footprint — check any old plans or probe carefully with a thin rod.
- Allow at least 500 mm clearance on all sides for safe working room.
3. Use the Right Equipment
- A mini excavator is ideal for most residential tank removals. Operators should work slowly and carefully near the tank itself.
- Switch to hand digging (spades and shovels) within 300–500 mm of the tank to avoid puncturing it.
- Never use a mechanical bucket to drag or strike the tank directly.
4. Watch for Contaminated Soil
- Keep contaminated soil separate from clean fill. It cannot go to a standard landfill — it must be disposed of at an approved contaminated waste facility. Your regional council can advise on approved sites.
- Wear appropriate PPE: nitrile gloves, safety boots, and eye protection at minimum. If contamination is present, a P2 respirator is advisable.
5. Expose the Tank Fully Before Lifting
- Fully expose the tank on all sides and underneath before attempting to lift it.
- Check for any remaining pipe connections and disconnect them safely.
- Use appropriate lifting gear — even a small steel tank can weigh several hundred kilograms.
Disposal and Backfilling
Once removed, the tank must be disposed of responsibly. Many scrap metal dealers will accept a cleaned, degassed steel tank. Confirm with your local dealer first.
Backfill the excavation with clean, compacted fill. If contaminated soil was removed, you may need to import certified clean fill and potentially have the site tested again before signing off. BRANZ has useful resources on site remediation best practice.
When to Call a Professional
In most cases, you should engage qualified professionals for a buried oil tank removal. This is not a job to cut corners on. You'll likely need:
- An environmental consultant if contamination is suspected
- A licensed fuel technician to drain and degas the tank
- A qualified excavator operator for the dig
- Possibly a licensed building practitioner if the tank is under or near a structure — check with LBP
Note that any work involving gas connections is restricted work under the Gas Act and must only be carried out by a licensed gasfitter registered with the Plumbers Gasfitters & Drainlayers Board. Similarly, any electrical disconnections near the tank must be handled by a registered electrician — see the Electrical Workers Registration Board.
You can find experienced local excavation contractors and environmental specialists on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
