Sydney's mothballed desalination plant has been reactivated for the first time in almost seven years as Sydney's water levels dropped below 60 per cent on Sunday morning.

Sydney Water confirmed the Kurnell facility was restarted on Sunday, with the city's water reservoir's now sitting at 59.9 per cent.

The Sydney Desalination Plant has been turned on due to dam levels dropping below 60 per cent.

The plant, which filters salt molecules from sea water, will take up to four months to produce drinkable water, and twice that time to be running at its maximum output of 250 million litres a day - about a sixth of Greater Sydney's needs.

In a statement on its website, Sydney Water said water from the desalination plant must meet the Australian Drinking Water Guidelines.