Written by JSW Plumbing | Published on July 2025
A sudden jump in your water bill, with no obvious dripping tap or wet patch in sight, is almost always caused by a hidden leak, a continuously running toilet, or a faulty meter. In the Hornsby, Thornleigh and Hills District area, we see this issue regularly, especially in homes built before the 1990s with older plumbing systems.
This article walks you through the most common culprits, how to check for them yourself, what it might cost to fix, and exactly when you need a licensed plumber on site.
Understand how much water a hidden leak can waste
Before you panic about your bill, it helps to understand just how much water small, invisible leaks can waste. The numbers are often shocking.
A toilet with a faulty flapper valve can leak 200 to 400 litres per day without making any noise. A pinhole pipe leak behind a wall can lose 150 litres per day. Over a 90-day billing quarter, that adds up fast.
| Leak Source | Estimated Daily Loss | Quarterly Loss (90 days) |
|---|---|---|
| Running toilet (faulty flapper) | 200 to 400 litres | 18,000 to 36,000 litres |
| Dripping tap (1 drip/sec) | 10 to 15 litres | 900 to 1,350 litres |
| Pinhole pipe leak | 100 to 200 litres | 9,000 to 18,000 litres |
| Faulty irrigation zone | 300 to 600 litres | 27,000 to 54,000 litres |
| Hot water system pressure valve | 50 to 100 litres | 4,500 to 9,000 litres |
Sydney Water charges residential customers roughly $2.35 per kilolitre for the first tier of usage. Even a modest pinhole leak could add $40 to $80 to your quarterly bill without you ever seeing a wet floor.
Check your water meter before calling anyone
The first thing to do is confirm whether the high bill is a billing error or a real usage problem. Your water meter gives you this answer in under five minutes.
- Turn off every tap, appliance, and irrigation system in your home.
- Locate your water meter, usually near the front boundary of your property.
- Write down the current meter reading, including the small red or black dial.
- Wait 30 minutes without using any water.
- Check the meter again. If the dial has moved, water is flowing somewhere it should not be.
If the main dial moves even slightly during that 30-minute window, you have a leak somewhere on your side of the meter. This means you are responsible for fixing it, not Sydney Water.
If the meter does not move, contact Sydney Water to query the bill directly. Billing errors do happen, though they are less common than actual leaks.
Identify the most common hidden leak sources in Hills District homes
In Thornleigh, Hornsby, and across the Hills District, certain plumbing problems appear more often than others. Older homes with copper or galvanised steel pipes are particularly prone to pinhole corrosion leaks inside walls and under slabs.
Here are the places you should check first:
- Toilet cisterns: Drop a few drops of food colouring into the cistern. Wait 10 minutes without flushing. If colour appears in the bowl, the flapper is leaking silently.
- Hot water system: Check the pressure relief valve on the side or top of your unit. It sometimes drips into a drain pipe outside, making it invisible unless you look closely.
- Irrigation and garden systems: A stuck solenoid valve can run a zone continuously overnight. Check your controller settings and inspect any buried drip lines.
- Under-slab pipes: Warm patches on tile floors, soft spots in carpet, or the sound of running water in a quiet house all suggest a slab leak.
- Meter to house supply line: The pipe running underground from your meter to the house can crack or separate over time, especially after dry summers followed by heavy rain.
JSW Plumbing frequently gets called out in Thornleigh and the surrounding suburbs to diagnose exactly these issues. A lot of customers have already tried to find the source themselves and come up empty handed before calling us.
Compare repair costs for common hidden leaks in Sydney
Knowing the likely repair cost helps you make an informed decision about whether to investigate further or get a plumber in straight away. Prices below are general estimates for the Sydney metro and Hills District area and may vary depending on access and the specific plumber you engage.
| Repair Type | Estimated Cost (AUD) | DIY Possible? |
|---|---|---|
| Toilet flapper replacement | $80 to $180 | Yes, with moderate skill |
| Tap washer or cartridge replacement | $100 to $220 | Limited (licence required in NSW) |
| Pressure relief valve replacement | $180 to $350 | No, licensed plumber required |
| Pipe repair inside wall or ceiling | $400 to $1,200 | No |
| Under-slab leak detection and repair | $1,500 to $4,500 | No |
| Irrigation solenoid valve replacement | $150 to $300 | Sometimes |
| Supply line replacement (meter to house) | $1,200 to $3,500 | No |
In NSW, most plumbing work beyond replacing a toilet seat or shower head requires a licensed plumber. Attempting unlicensed plumbing repairs can void your home insurance and create liability issues.
Know when your usage genuinely increased without a leak
Sometimes the bill is high because water usage actually increased. It is worth ruling out lifestyle changes before assuming you have a leak.
- New people in the household, such as a returning student or family member moving in.
- Filling a swimming pool or spa for the first time this season.
- Running irrigation more frequently during a hot Thornleigh summer.
- A dishwasher or washing machine running more cycles than usual.
- A house guest who takes long showers or leaves taps running.
- A new garden being established that requires daily watering.
If none of these apply and your meter test showed movement, the problem is almost certainly a hidden leak. Hills District summers can be brutal, and many households unconsciously use more water during heat waves, but not enough to double a bill.
Understand slab leaks and why they are serious in older Thornleigh homes
Slab leaks are one of the more serious and expensive hidden leak types. They occur when a pipe running beneath your concrete slab foundation develops a crack or joint failure.
Homes built in Thornleigh and Hornsby during the 1960s through to the 1980s often have copper or galvanised pipes embedded in or under the slab. These materials corrode over time, particularly in areas with reactive soil, which is common throughout the Hills District.
Signs that you may have a slab leak include:
- Warm or hot spots on tile or concrete floors (hot water line leaking).
- The sound of water running when everything is turned off.
- Cracks appearing in internal walls or floor tiles with no obvious cause.
- Unusually lush or wet patches in the garden directly above the slab edge.
- A water bill that has been creeping up over several quarters, not just one.
JSW Plumbing uses electronic leak detection equipment to locate slab leaks without unnecessary excavation. Identifying the exact location before digging saves significant time and cost.
If you suspect a slab leak in your Thornleigh or Hills District home, do not ignore it. Water under a slab can erode soil, undermine footings, and cause serious structural damage over time.
FAQs on high water bills with no visible leak
Can a running toilet really cause a bill to double?
Yes, absolutely. A faulty toilet flapper that allows a constant trickle can waste 200 to 400 litres per day. Over a 90-day billing quarter, that is up to 36,000 litres of extra water, which can add $80 or more to your bill depending on your usage tier with Sydney Water.
Who is responsible for fixing a leak between my meter and my house?
In NSW, Sydney Water owns and maintains the infrastructure up to and including your water meter. Any pipe or fitting on the house side of the meter is your responsibility as the property owner. A licensed plumber like JSW Plumbing can repair or replace the supply line and help you document the leak for any possible Sydney Water rebate application.
Can I get a rebate from Sydney Water if a leak caused a high bill?
Sydney Water does offer a leak allowance in some circumstances, but it is not automatic. You generally need to show that the leak has been repaired by a licensed plumber and provide the invoice as evidence. Contact Sydney Water directly after the repair to request an assessment of your account.
How long does leak detection take for a hidden pipe leak?
A basic meter test takes about 30 minutes and you can do it yourself. Professional electronic leak detection for a suspected in-wall or under-slab leak typically takes one to two hours on site. JSW Plumbing can usually attend within one to two business days for non-urgent investigations in the Hornsby and Thornleigh area.
My hot water system is old. Could that cause a high water bill?
Yes. Hot water systems over 10 years old commonly have failing pressure and temperature relief valves that discharge water continuously into a drain. They can also develop internal tank leaks that are not visible externally. If your hot water system is more than 10 to 12 years old, it is worth having a plumber inspect it, especially if your bill has spiked unexpectedly.
Is it worth paying for leak detection or should I just fix the most likely cause first?
If your meter test confirms a leak but you cannot find the obvious source after checking toilets, taps, and your hot water system, professional leak detection is worth the cost. Guessing wrong and opening the wrong wall or digging the wrong part of your yard costs far more than a detection service. Detection fees in the Hills District area typically range from $200 to $450.
Wrap up and next steps
A suddenly high water bill almost always has a fixable cause. Start with the meter test, check your toilets and hot water system, and look for the signs of slab or in-wall leaks described above.
If you have checked the obvious sources and still cannot find the problem, the team at JSW Plumbing can help. We serve Thornleigh, Hornsby, and the wider Hills District with professional leak detection and repair using the right equipment to find the problem fast.
Call JSW Plumbing today to book a leak investigation or request a quote. The sooner the leak is found, the sooner you stop paying for water you are not using.