The Water Corporation could be forced to take its full allocation from the ailing Gnangara groundwater system for the third year as Perth heads towards its driest July on record.
The utility said yesterday it was too early to tell whether it would need to ask the Department of Water for up to 165 billion litres from the Gnangara but an unprecedented dry spell could leave it with little choice.
The Weather Bureau said Perth had just 27.8mm of rain so far this month, significantly short of the lowest total recorded for July of 61.5mm in 1876. The average for July is 169.6mm.
Showers are forecast for early next week but they are not expected to be heavy.
Perth's dams are below 30 per cent of capacity and hold only 70 billion litres of useable water, prompting the Water Corporation last week to plead with customers to use less of the precious resource.
The lack of rain leaves the State-owned group poised to seek more water from the Gnangara given that groundwater from the system still provides about half of Perth's drinking supplies.
It also comes despite WA's second desalination plant coming on stream at Binningup in September and amid work costing $450 million to double the facility's capacity to 100 billion litres a year.
A spokeswoman for the utility said the expansion was still scheduled to be completed by the end of the year, by which time it was hoped Perth's dams would be boosted by late rains.
She also said the corporation would continue to focus on efforts to reduce per-capita water consumption while pushing ahead with plans to recycle highly treated sewage.
However, the spokeswoman could not rule out having to ask for an increased allocation from the Gnangara aquifer if there was not enough rain.
"It is far too early to tell what final groundwater allocation the Water Corporation will need to apply for from the Department of Water this year," she said.
Conservation Council of WA director Piers Verstegen noted the corporation's move to tap deeper parts of the aquifer rather than more sensitive shallow parts, but said it was alarming the Gnangara was being used as a "backstop".
"We've got very little confidence in the (State Government's) ability to manage the Gnangara in a sustainable way," Mr Verstegen said.