The equivalent of an extra 17,600 Olympic-sized swimming pools of water will be taken from Perth's aquifers this year, as a prolonged dry spell bites hard on supplies.

The Department of Water has approved the Water Corporation's application to take 165 gigalitres from the Gnangara and Jandakot aquifers.

That is 44 gigalitres, or 44 billion litres, more than last year, though about four gigalitres less than in 2006-07, the last prolonged dry period.

The majority of the drawdown, 151.71 gigalitres, will be from the fragile Gnangara aquifer.

The Water Department's water resource director, Rob Hammond, said the request had been agreed to only after the Water Corporation showed it had maximised the output of the Kwinana desalination plant and ensured demand management measures during dry spells remained in place.