Water Tank Leaking? Causes, Fixes and When to Call a Professional

A leaking water tank wastes the very resource it’s meant to store, and if it’s left too long it can undermine the tank’s structure, your stand or slab, and the ground around it. This guide walks through why tanks leak, how to track a leak down, what you can fix yourself, and when the job needs a professional.

Why is my water tank leaking?

Most water tank leaks come down to five causes: perished seals, corroded fittings, structural cracks, poor pipe connections, or overflow issues. Which one applies to you usually depends on your tank material.

Corroded or worn fittings

Outlet valves, tank taps, and pipe connections are usually the first point of failure. Rubber seals and washers dry out and crack over time, especially in tanks exposed to full sun. A slow drip at a fitting is one of the easiest leaks to spot and, often, one of the cheapest to fix.

Cracks in concrete tanks

Concrete tanks develop hairline cracks as the structure settles or as the concrete cures unevenly over the years. These start as damp patches on the outside wall and can widen if left untreated, eventually letting water through steadily rather than just seeping.

Splits or punctures in poly (plastic) tanks

Poly tanks can crack from UV degradation, impact damage, or stress at the base where the tank’s full weight sits. A puncture from a falling branch or a knock during cleaning is also common.

Rust in steel or corrugated iron tanks

Older galvanised and corrugated iron tanks rust from the inside out. Once rust breaks through the metal, patch repairs only buy time. This is usually a sign the tank is nearing the end of its working life.

Overflow and inlet pipe issues

Sometimes what looks like a tank leak is actually water escaping around an overflow pipe or inlet that wasn’t sealed correctly during installation. Check these connection points before assuming the tank body itself is at fault.

How do you find a leak in a water tank?

Start by inspecting the tank when it’s full and the ground is dry, since that’s when leaks are easiest to trace to their source.

1. Check the outside walls for damp patches, streaking, or moss growth, which show where water has been tracking down over time.
2. Inspect every fitting: tap, outlet valve, overflow, and inlet, for drips or corrosion.
3. Look at the base for pooling water, soft or eroded ground, or a damp concrete stand.
4. Mark the water level and check it again 24 hours later (with no usage) to confirm the tank is actually losing water rather than it being used elsewhere in the system.
5. If nothing is visible from outside, the leak may be underground or at the base seam, and this is where a professional inspection is worth calling in.

How do you stop a plastic water tank from leaking?

For small cracks or pinhole punctures in poly tanks, a purpose-made poly tank repair kit or two-part epoxy designed for potable water contact can seal the area from the inside or outside, depending on access.

The surface needs to be completely dry and cleaned of algae or grime first, or the patch won’t hold. Larger splits, especially anything running along a stress line near the base, usually need a professional weld or a full panel replacement rather than a patch.

How do you fix a leaking concrete water tank?

Hairline cracks in concrete tanks are typically repaired with a cementitious waterproofing compound or crystalline sealant that bonds into the concrete and expands to close the gap from within, rather than just sitting on the surface like a paint-on sealer. Wider structural cracks, especially ones that have grown or that appear alongside cracking in the tank stand or slab, are a sign of movement in the structure and need a proper structural assessment, not just a surface patch.

Is a leaking water tank dangerous or urgent?

A slow drip at a fitting isn’t an emergency, but it should still be fixed before it wastes more water or the fitting fails completely. Leaks are more urgent when: the tank is losing a visibly large volume, water is pooling near a tank stand or slab, the tank services fire-fighting or stock water supply, or you can see the crack widening. In these cases, treat it the same as any other trade emergency and get someone out promptly rather than waiting.

DIY fix or call a professional?

As a general rule:

• DIY-friendly: a dripping tap or valve washer, a small surface crack in poly with a repair kit, resealing an overflow pipe fitting.
• Call a professional: structural cracks in concrete, rust-through on steel tanks, splits near the tank base, underground tank leaks, or any leak you can’t locate after a proper inspection.

Getting a professional inspection early is almost always cheaper than waiting for a small leak to become a full tank failure. Our team inspects, repairs, and where needed restores rainwater tank systems across NSW, covering everything from concrete tank repairs to underground water tank servicing.

How to prevent water tank leaks in future

Regular maintenance catches most leaks before they start. Keep on top of these basics:

• Inspect fittings and seals annually, or after extreme weather
• Keep the tank exterior clear of vegetation that traps moisture against the walls
• Clean debris from inlet screens and overflow points so water isn’t forced to find another way out
• Have filtration systems serviced so sediment build-up doesn’t put pressure on internal fittings
• Book a professional check-up as part of routine rainwater tank maintenance, ideally before peak rainy season

Frequently asked questions

Why is my water tank leaking from the bottom?

Leaks at the base are usually caused by cracking where the tank meets its stand or slab, a failed base seam on poly and steel tanks, or ground movement putting uneven pressure on the tank floor. This is one of the more serious leak locations, since it’s harder to patch and often points to structural movement rather than a simple seal failure.

Can I repair a water tank myself?

Small, accessible leaks like a worn tap washer or a hairline poly crack can often be fixed with the right kit and a bit of patience. Anything structural, underground, or affecting a large section of the tank wall should go to a professional, both for safety and to make sure the repair actually holds under the full weight of a refilled tank.

How much does it cost to fix a leaking water tank?

Cost depends on the tank material, the size and location of the leak, and whether it’s a simple seal replacement or a structural repair. A dripping fitting is a minor cost; a structural concrete repair or a steel tank nearing end of life is a bigger job. Get an inspection first so you’re quoted for the actual repair needed, not a guess.

Will a leaking tank affect my water quality?

It can. A crack or gap that lets water out can also let contaminants, insects, or debris in, which affects the water sitting inside the tank. If you’ve had a leak for a while, it’s worth having your water tested once the repair is done.

How often should I check my water tank for leaks?

An annual inspection is the minimum, with an extra check after major storms, heatwaves, or if you notice a drop in expected water levels. Tanks under heavier use, like those feeding irrigation or stock water, are worth checking more frequently.

Do all water tanks eventually leak?

All tanks age, and fittings, seals and even tank walls will eventually wear down regardless of material. Regular maintenance won’t stop this permanently, but it catches the early signs and extends the tank’s working life significantly compared to a set-and-forget approach.

Get your leaking tank inspected

If you’ve spotted a leak, or you’re not sure whether what you’re seeing needs a repair, our team can inspect, diagnose, and repair rainwater tanks across Sydney, Canberra, and the surrounding regions. Get a quote or call 1800 702 156 to book an inspection.