Leaking Concrete Tank Repair
Is your concrete water tank leaking? You're not alone — it's the single most common reason we're called out, and the fix is more straightforward (and cheaper than replacement) than most people expect.
Here's the truth: a leaking concrete tank is almost never "broken." It's a tank that's reached the end of life of its original waterproofing, and it can be brought back to full function — for decades — without digging it out and starting again.
Why your concrete tank is leaking (the real reason)
Most leaking concrete rainwater tanks fail the same way:
Rainwater sits against the internal plaster or bare concrete surface. Over time the water (which is naturally low in calcium) leaches the lime out of the cement.
As the cement binder dissolves, the sand aggregate falls away, layer by layer, until the steel reinforcing inside is exposed.
The steel gets wet and rusts. Rust (oxide jacking) expands up to 10x the original volume of the steel — and that expansion is what cracks the sidewall, usually in straight lines following the reinforcing.
That's why patching from the outside rarely lasts. You can't cut the rust out of a water tank, so any plaster or epoxy over it just gets pried apart again by the same rust jacking. It holds for a while — then the leak comes back.
See more info on this in our blog post.
The permanent fix: a geomembrane tankliner
A custom geomembrane tankliner is the only long-term solution for a leaking concrete tank. Here's why it works where patching doesn't:
It's not glued to the failing surface. The liner is fixed at the top of the sidewall only and held against the walls and floor by the water itself. That means it bridges cracks and voids — and keeps bridging them even if the crack moves.
Our membrane stretches to 1000% before tearing. The Permaflex fPE500 polyethylene geomembrane we use is rated to 1000% elongation at break — a 1mm crack would need to become a 10mm crack before it's a concern.
It stops the damage at the source. Once fitted, the liner removes fresh water from contact with the concrete entirely, halting further degradation and extending the life of the whole system.
Certified safe to drink from. All our material is certified to AS/NZS 4020:2005 for potable water contact — no BPA, no taste, colour or odour imparted to your water.
How long does it take?
For domestic tanks, most Tankliners are installed in under 5 hours and we can often book a water truck for the same day. We prefabricate every liner in our Auckland workshop before we arrive, along with as much prep as possible done in the workshop to minimise time on site (time without water!).
What does a leaking tank repair cost?
For a ‘standard 5,000 gallon’ (≈22,500L) concrete tank — the size most 20–40 year old domestic tanks are — a full Tankliner supply and install is usually $2,900 +GST (~$3,300 incl GST). That includes emptying and cleaning the tank, replacement outlet fittings & isolation valve, and pressure-sealed overflow pipework.
We work on a fixed-quote basis and always price up any extras (additional penetrations, internal suction pickups, etc.) upfront — no surprises. Larger cuboid or subfloor tanks are quoted to size.
Every install carries a 10-year pro-rata warranty, and the liner itself is rated for a 30–50 year service life. Compare that to replacement — a new tank plus removal and refit can run several times the cost, with more downtime.
We've seen it all
From 100-litre tanks to 1,000,000-litre fire reserves, from golf-ball-sized holes in a sidewall to subfloor tanks with no external access — we've lined 500+ tanks, and we've only ever had to delay a refill once. If your tank can be saved, we'll tell you straight. If it genuinely isn't worth spending money on, we'll be brutally honest about that too.
We hand-make every single tankliner we install. No two tanks are the same, so neither are our liners.
Service area
We're based in Auckland CBD and cover all of wider Auckland (Pukekohe to Warkworth to Waiheke), plus we travel for jobs all around the country — Northland, the Coromandel, Waikato, Bay of Plenty, Hawke's Bay, Taranaki, Manawatu, Wellington, and the South Island from the Sounds to Invercargill.
Get your tank fixed — for good
Call Kieran on +64 20 499 3555, or request a quote via the button below and we'll get back to you fast. Much of what we do is urgent, and we keep a range of common tankliner sizes on hand for those 6pm phone calls.