The Berejiklian government will fast-track an expansion of Sydney's desalination plant, which will double it in size to provide more than 30 per cent of the city's drinking water.

With dam levels dropping to 43 per cent on Wednesday, NSW Water Minister Melinda Pavey has directed the operators of the plant to prepare for an expansion "as quickly as practicable".

NSW Water Minister Melinda Pavey has directed that planning start on the expansion of the desalination plant. Credit:Christopher Pearce

Ms Pavey said the expansion of the plant in Kurnell, in Sydney's south, was a "key element in protecting Sydney's water security".

"The expansion of the plant should be undertaken as quickly as practicable and in a prudent and efficient manner to deliver at least an additional 250 megalitres of drinking water per day averaged over a 12-month period," Ms Pavey said.