Sydney households will have up to $35 added to their annual water bills to cover the restart of the city's desalination plant, the Berejiklian government says.

On Tuesday, Sydney and Wollongong's dams were 60.3 per cent full, marginally above the trigger point for switching on the desalination plant. On current projections, the sub-60 per cent mark is likely to be crossed this weekend.

An employee of the Sydney Desalination plant walks past some of its 36,000 polymer membranes used to filter salt and other impurities from seawater so that it is suitable for drinking.

A spokesman for Don Harwin, the urban water minister, said households would pay as much as $35-a-year extra to cover the plant's operations.

He also said the public should follow the Water Wise rules as promoted by Sydney Water. These include avoiding watering lawns and gardens between 10am and 4pm, only cleaning cars, boats or buildings using a bucket, high-pressure cleaning equipment or a hose filled with a trigger nozzle.