Welding a cracked steel water tank yourself is technically possible if you have solid welding experience — but it is not a job to take lightly. There are genuine explosion and fire risks involved, and depending on how the tank is used, there may also be health and building compliance considerations. Read this guide carefully before picking up a welder.
Why This Job Is More Dangerous Than It Looks
The biggest hazard with welding any tank — even one that has held only water — is the risk of confined-space explosion or fire. A tank that has been emptied can still contain vapour pockets, rust gases, or residue that ignites when exposed to welding heat or sparks. This is a well-documented cause of serious injuries and fatalities in trade settings.
Even a tank used purely for rainwater or cold potable water can accumulate organic matter and gases over time. Never assume an empty tank is safe to weld without proper preparation.
WorkSafe NZ has clear guidance on confined space and hot-work hazards. Review their resources before starting any tank welding project.
Is It Restricted Work Under NZ Law?
Welding itself is not a licensed trade in New Zealand the way electrical or plumbing work is. However, the use of the tank matters:
- Potable (drinking) water tanks: If the tank supplies drinking water, any repair work must not introduce contaminants. Materials and processes must be food-safe. Check with your local council and consider whether the repair needs to comply with NZ Building Code clause G12 (water supplies).
- Plumbing connections: If the crack is near or involves a pipe fitting, valve, or any plumbing connection, that work is restricted to a licensed plumber under the Plumbers, Gasfitters and Drainlayers Act. See PGDB for what counts as restricted plumbing work.
- Rural or fire-fighting tanks: Generally fewer restrictions, but structural integrity still matters.
Always confirm with your local council whether a building consent is required for repairs or replacement. Some tank installations fall under Schedule 1 exemptions, but it pays to check — councils' interpretations can vary. See MBIE Building Performance for general guidance on consent requirements.
Before You Start: Essential Safety Steps
If you decide to proceed yourself, follow these steps without shortcuts:
- Fully drain and flush the tank — remove all water, sediment, and debris.
- Ventilate thoroughly — open all access points and allow the tank to air out for an extended period (at minimum several hours, ideally longer for larger tanks).
- Test the atmosphere — ideally use a gas detector to confirm no flammable or toxic gases are present before welding. This is standard trade practice.
- Never weld a sealed tank — always ensure there is an open vent so pressure cannot build up from heat.
- Keep a fire extinguisher within arm's reach and have a second person present as a safety observer.
- Wear appropriate PPE — welding helmet with correct shade, leather gloves, flame-resistant clothing, and steel-capped boots.
Assessing the Crack: Is It Even Weldable?
Not all cracks are equal. Consider:
- Crack location: A crack near a weld seam, fitting, or structural stress point may indicate a deeper problem with the tank's integrity.
- Tank wall thickness: Thin-gauge steel (common in domestic tanks) can warp, blow through, or distort easily if you are not experienced with low-heat welding techniques.
- Extent of corrosion: If the steel around the crack is heavily rusted or pitted, welding may not hold and the tank may need replacement rather than repair.
- Tank age and condition: An old tank with one crack often has other weak spots developing. A repair may only delay the inevitable.
If in doubt, have a qualified welder assess the tank first. A professional inspection is far cheaper than a failed repair or a flooded property.
Suitable Welding Processes
For steel water tank repairs, common approaches include:
- MIG (GMAW) welding — good for thinner steel, easier for DIYers with some experience.
- Stick (SMAW) welding — more versatile outdoors or in awkward positions, but requires more skill on thin material.
- TIG (GTAW) welding — highest quality finish, but requires significant skill and is usually left to professionals.
Use a low-hydrogen electrode or wire suitable for mild steel, and ensure your filler material is appropriate for potable water contact if the tank stores drinking water.
When to Call a Professional
Call a qualified welder or engineer if:
- You have no prior welding experience — this is not the job to learn on.
- The tank is large, pressurised, or elevated (structural failure risk).
- The crack is extensive, near fittings, or accompanied by corrosion.
- The tank supplies drinking water and you are unsure about material compliance.
- Any plumbing connections are involved — this is restricted work requiring a licensed plumber.
- You cannot safely ventilate and test the tank atmosphere.
A qualified local welder can assess whether repair is worthwhile or whether replacement is the smarter investment. You can find experienced welders and tradespeople in your area on tradedirectory.co.nz.
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