Perth's dams have almost stopped supplying the city and broader South West with drinking water earlier than predicted five years ago in a worst-case scenario.
As the Weather Bureau said dry, sunny winters such as the current one in Perth were fast becoming normal, the Water Corporation laid bare the extent to which the city depends on desalination plants.
Former Water Corp boss Jim Gill warned in 2008 that under extreme circumstances Perth's dams could become redundant by 2015 but the utility said yesterday the changes were already happening.
In a series of blunt answers, the corporation said the only thing that had prevented the dams being taken offline in recent years was the desalinated water and groundwater used to top them up.
Under the corporation's policies, water in its metropolitan dams is deemed "unusable" once storage levels fall below 110 billion litres, largely because of the concentrations of organic matter left over.
As of yesterday, the Water Corp's dams were at 25 per cent capacity and holding 154 billion litres, the third-lowest level on record behind July 2007 and July 2011, when they were at about 24 per cent.
A corporation spokeswoman said it was difficult to determine how much of the dams' current levels came from desalinated water and groundwater pumped there for storage.
However, she noted practically all storage this year was "banked" from other sources and about 50 billion litres had been transferred into them last year - suggesting the dams would have been taken offline without it.
"Without desalination and groundwater our dams would be extremely low," the spokeswoman said.
"Last year, we received only 18 billion litres of inflow to dams - consumption was 279.3 billion litres, so groundwater and desalination is vital.
"Rainfall is no longer relied upon as our primary source of water. Our investment in desalination has secured our water supply in dry years."
The Weather Bureau said dry winters like this year's were likely to become the rule rather than the exception.
Perth recorded its second driest June on record this year - when just 44mm of rain fell compared with the average of 175mm - and senior forecaster Neil Bennett said it continued a "disturbing" trend.
"Four of the five driest years have all occurred since 2006," Mr Bennett said. "At what point does that stop being strange and start becoming normal?"
A run of cold mornings in Perth continued yesterday, with the temperature dropping to 3.2C. Today's minimum is predicted to be 9C before dropping as low as 2C later in the week.