Adrian Strooper lives in Australia's wettest major city, but he still thought it prudent to install a rainwater tank when he was renovating his home. 

Mr Strooper only uses the 5,000-litre tank to top up his garden; the relatively low cost of town water in Cairns makes it uneconomical to harvest rainwater for his household needs.

That cost equation might soon have to change.

Cairns has an average annual rainfall of 1,992mm, but, despite the abundance of water, its drinking supply has grown increasingly precarious in recent years.

The city was said to be hours from running out of potable water in December when debris blocked intakes at its only water treatment plant after cyclone Jasper hit the city.

The damage and disruption to supply forced Cairns into water restrictions for three weeks.

While nearby Townsville has two dams and four water treatment plants, Cairns's water supply relies entirely on Copperlode Dam, built in 1976 for a population a third of the city's current size.

The Cairns Regional Council is about to embark on a $472 million project to augment its supply with a new intake and treatment plant to avoid the risk of supply shortfalls it says could happen in as little as two years.

A thirsty city

The need for such an expensive water security project, which had a $224 million cost blowout before it even began, has thrust Cairns's water use efficiency in the spotlight.

On average, each residential property in Cairns used 249,000 litres of water in the year to June 2023 — significantly more than all other major Australian cities, except Darwin and Townsville.

Cairns's wet-and-dry monsoonal climate makes comparisons difficult, but in the same period, the five water utilities in South-East Queensland supplied an average of 154,800 litres of water a year to each residential property.

"Our water use has come down a lot since the 1990s when we introduced water pricing and metering, but we think there is still a lot of work that can be done," said Bronwyn Opie from the Cairns and Far North Environment Centre.

A 2015 council strategy identified several ways the city could reduce its demand for water, including a scheme to help retrofit showers, toilets, and swimming pools in tourist accommodation to be more efficient.

"Only a handful of the 19 actions that were outlined in that strategy were ever initiated," Ms Opie said.

She said the council should also work directly with the city's largest water users to help address any inefficiencies.

"Fifty per cent of our metered use in Cairns is billed to only 10 per cent of accounts, so that means there's a small handful of people and businesses that are using a lot of our water," Ms Opie said.

Water bills set to rise

The need to find more water savings becomes more important for Cairns Regional Council as it scrambles to get together the money it needs for the first stage of the water security project.

A contract has already been awarded and work is due for completion in 2026.

This week, the Queensland government topped up its already hefty contribution with an extra $87.5 million, but a similar amount is still needed from the Federal government.

Mayor Amy Eden said even with state and federal funding to help make up the shortfall, Cairns ratepayers would still be slugged an extra $85 each year on their water rates to cover the cost of the project.

Cairns's population is expected to grow by 80,000 by 2041 and further stages of water security infrastructure, including a new dam, would eventually be required.

"Demand management offers opportunities for us to defer stage 2 and 3 and obviously what comes with that is deferring the major capital expenditure that comes with these large infrastructure projects," Ms Opie said.

Advocates have also argued residential rainwater tanks — which the council considers "not effective for Cairns" because of the city's months-long dry season — should play a greater role in addressing the problem.

Mr Strooper said he would like to install the pump and filtration systems he would need to use tank water for some of his household needs, but the costs do not yet stack up.

The other challenge is the concentration of rainfall across just four months of the year.

"Using it for my garden, just off and on trickle stuff … [a full tank] would probably last me four or five days," he said.

"It's more just a top-up, as you need it, for extras."