Planting local native gardens at every Perth and South West home instead of exotic plants and lawns could save 45 billion litres of drinking water a year.
Amid rocketing water prices, the Water Corporation estimates the equivalent of the entire annual output of the Kwinana desalination plant is used to keep European-style gardens lush.
The revelation prompted the State-owned utility and gardening experts to urge more householders to plant local species for financial as well as environmental reasons.
It also came as new figures showed ritzy western suburbs such as Peppermint Grove and Dalkeith continued to top the list of Perth's biggest water-guzzling suburbs.
Along with suburbs on Perth's fringes and in the Perth Hills, Peppermint Grove and Dalkeith homes on average used up to 439,000 litres of scheme water in the 12 months to June 30.
In comparison, the typical household used about 250,000 litres, while homes in suburbs such as East Perth, Rockingham and Fremantle consumed as little as 134,000 litres.
Ben Jarvis, the corporation's water efficiency manager, said residential customers used 71 per cent of Perth's drinking supplies and 42 per cent, or 90 billions litres, of this went on to gardens.
Mr Jarvis said although some local native plants used more water than others and not all exotic species were thirsty, in general gardens of waterwise local plants needed about half as much water.
While noting Perth's per capita water consumption had dropped 26 per cent in 10 years, Mr Jarvis said "there are many things customers can do in the garden to reduce their water use".
These included improving their soil, planting species that suited the climate, mulching the garden properly and choosing an efficient sprinkler system.
Pat Madden said she replanted her North Perth garden with native species because they were easier to care for - and the move had slashed her water bills.