Perth's desalination plant has been a lifesaver but debate rages around the value of Sydney's plant which sits broken and idle.

Western Australia's capital now relies on desalinated seawater for half of its water due to its climate clearly drying and its population definitely rising.

Rainfall run-off into Perth dams had dropped by 90 per cent over the past century.

During the Millennium drought many other Australian cities moved to shore-up supply by commissioning multi-billion-dollar desalination plants.

Adelaide uses its plant, Melbourne fired up its seldom utilised resource last year, the Brisbane-Gold Coast plant is on permanent standby, but Sydney's remains idle after being damaged in 2015.

The repairs to Sydney's plant will be completed by December 2018 but critics nonetheless say it was a $2 billion "financial disaster" built in a panic.

Climate scientists and some water experts defend desalination plants as vital insurance that will be needed sooner or later.

Perth's drying climate identified early

Climate scientist Pandora Hope worked on the Indian Ocean Climate Initiative from 1998 where she identified that Perth's winter rains were failing.

It fell away by a remarkable 20 per cent just in the 1960s.

Low levels plague Mundaring Dam, south of Perth, after decades of declining rainfall. ( ABC Rural: Bill Bunbury )

"We stayed at those lower levels, then in the 1990s we seem to have seen another drop," Dr Hope said during last week's 2018 Australian Meteorological and Oceanographic Society (AMOS) conference in Sydney.

"I guess the really important thing is the global coupled climate models really align with this signature.

"For rainfall globally, Perth is one of the most consistent and robust changes we see in the global climate models."

Dr Hope says the drying climate will continue in south-west WA.

"The global climate models are providing a really consistent signal for what we've observed"… It's an ongoing decline, particularly winter rainfall.

"Summer rainfall is a different story as tropical influences start to impact the area a bit more, but those won't really make up the shortfall for what's happening in winter."

Mediterranean climates drying out globally

The data on Perth is mirroring similar Mediterranean climates around the world.

You're drinking seawater: Perth relies on half its water supply, 149.8 billion litres in 2016-17, from two desal plants built in 2006 and 2012

relies on half its water supply, 149.8 billion litres in 2016-17, from two desal plants built in 2006 and 2012 Adelaide's desal plant (2012) can supply 100GL of water a year, half the city's supply.

desal plant (2012) can supply 100GL of water a year, half the city's supply. Gold Coast's desal plant (2009) operates on a 'hot standby' mode, part of the grid which connects Brisbane and other south-east Queensland cities

desal plant (2009) operates on a 'hot standby' mode, part of the grid which connects Brisbane and other south-east Queensland cities Victoria's plant (2012) can supply a third of Melbourne's water

plant (2012) can supply a third of Melbourne's water Sydney's plant (2010) was damaged in 2015 and is in maintenance mode. It costs $85 per year on householders' bills, $173 million this year.

"The rainfall has been declining in areas with Mediterranean-type climates around the world," said Dr David Karoly, the new leader of the CSIRO's climate change hub in the national environmental science program.

"Like Italy in the Mediterranean, south-west United States has also experienced rainfall decline.

"Areas like Las Vegas, increasing population (is coupled with) increasing water demand and decreasing rainfall.

"Los Angeles has had severe water supply issues, which has also been associated with warmer temperatures leading to accelerated snow melt."

Dry soils around Perth act like a huge sponge when it does rain.

"From 1911 to 1974, we averaged 338 gigalitres a year (streamflow into Perth dams), but 2010-2016 that averaged 42GL — roughly a 90 per cent decrease," said Dr Ian Wright, Environmental Science Lecturer at the University of Western Sydney.

The complexity of a desalination plant is both costly to run and mothball. ( ABC News )

"To me they are massive insurance policies, they can desalinate salt water pretty much at the flick of a button," Dr Wright said, writing in The Conversation.

"They're expensive, but what a fantastic insurance policy to have in your back pocket."

Dr Wright and climate scientists Dr Hope and Dr Karoly agreed that Sydney could return to water storage levels similar to 2007, below 30 per cent, a trigger for the desalination plant.

"We were going to face something like Cape Town and potentially the turning off at the mains, but we can supply a large percentage of our water," Dr Wright said.

"Particularly in Melbourne — that is a very, very large plant."

Sydney's plant 'a financial disaster'

Perth needed their desal plant, but NSW panicked in 2007 according to Professor Stuart Khan of the School of Civil and Environmental Engineering at the University of NSW.

He said the signs at the time were water storages would recover.

"We actually jumped the gun, it's been a financial disaster really," Professor Khan said.

"If you have a seawater desalination plant that's sitting there for a decade or two without being needed, that's more than $2 billion capital infrastructure.

"It's an insurance policy, but whether it's a good value insurance policy is a very different question."

