Why your water tank is leaking
Where the water appears tells you most of what you need to know. Look before you touch.
Most leaking tanks leak at a fitting, not through the wall. Check the outlet, the inlet and the overflow connection first, then the seams and the base. A crack in the tank body is the least common cause and the hardest to live with.
Start with where the water appears
Dry the outside of the tank and wait a day. Note the exact spot where it turns wet again. A leak low on the wall can start higher up and run down, so trace it back to the highest wet point.
- At the outlet or tap. A worn washer, a loose fitting or a failed seal.
- At a pipe entry. The tank fitting has loosened or the seal has aged.
- Along a seam or weld. Common on older steel tanks, often from rust.
- At the base. Water is sitting under the tank, or the base is not supporting it evenly.
Different materials fail in different ways
Steel tanks rust from the outside in or from the bottom up. Poly tanks rarely rust, but they can crack where a fitting puts stress on the wall. Concrete tanks can seep through hairline cracks.
| Tank type | Common leak point | Usual next step |
|---|---|---|
| Steel | Rust patches and seams | Repair or replace; rust spreads |
| Poly | Fittings and stress cracks | Replace the fitting; a cracked wall usually means replacing the tank |
| Concrete | Hairline cracks and joints | Specialist sealing |
| Any type | Base and overflow area | Check support and drainage |
A wet base is not always a leak
Condensation, overflow and rain splash all make the base wet. Before you blame the tank, check that the overflow is not discharging against it. See tank overflow causes for how to tell.
A poorly prepared base can also stress the wall and cause a crack over time. If the ground has settled under one side, the leak is a symptom. Tank base preparation explains what a good base looks like.
What you can check yourself
You can look, dry and watch. Mark the water level on a dry day and check it a day later with the pump off and no rain. A level that drops steadily points to a leak, not to use.
Do not patch a tank that holds drinking water with whatever is in the shed. Some sealants are not suitable for water you drink. When in doubt, treat the water as non-drinking until it is sorted.
How to find a slow tank leak step by step
A slow leak is easy to miss. This routine gives you a clear answer.
- Switch the pump off. Nothing should draw from the tank while you test.
- Pick a dry day. Rain can hide a drop in level.
- Mark the water level. Use tape or a pen on the outside if the tank is see-through, or measure from the lid.
- Check it a day later. A level that drops steadily points to a leak, not to use.
- Dry the outside and wait. Note the exact spot where it turns wet again.
- Trace back to the highest wet point. A low wet patch often starts higher up and runs down.
Write down what you find. The supplier or plumber can work from it.
Treating the water while the tank leaks
A leak lets contaminants in as well as water out. Until it is fixed, take a careful approach to the water.
| Use | Advice while it leaks |
|---|---|
| Drinking and cooking | Do not use it. Treat the water as non-drinking until the leak is fixed and the tank is cleaned |
| Toilets and laundry | Keep using it only if the connection is sound and a licensed plumber has not advised otherwise |
| Garden | Usually fine |
| Patching with whatever is in the shed | Avoid it. Some sealants are not suitable for water you drink |
| Fittings linked to the mains top-up, pump or house | Licensed work in every state and territory |
After the repair, read how to clean a rainwater tank, because a leak often lets dirt in.
Frequently asked questions
Why is my water tank leaking at the bottom?
Water at the base may come from a low fitting, a failed seam or an overflow discharging against the tank. Dry it, wait a day and find the highest wet point.
Can a leaking poly tank be repaired?
A loose or worn fitting can often be replaced. A cracked wall usually means the tank needs replacing, so ask the supplier.
How do I know if my tank is leaking or just being used?
Mark the level on a dry day with the pump off and nothing drawing from it. If the level still drops, it is leaking.
Is it safe to drink from a leaking tank?
A leak lets contaminants in as well as water out. Treat the water as non-drinking until the leak is fixed and the tank is cleaned.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NHMRC. Australian Drinking Water Guidelines
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)