The expected resumption of the Sydney Desalination Plant on Sunday will sharpen water as a prominent election issue, with Labor, the City of Sydney and the Greens stepping up criticism of the Berejiklian government's handling of the vital resource.

WaterNSW is likely to report the city's dam levels have dropped below the 60 per cent mark that automatically triggers an order for the $2.3 billion to be activated. It has been dormant since mid-2012 and will take as long as eight months to reach full capacity of 250 million litres a day - or about one-sixth of Sydney's water use.

The Sydney Desalination Plant will be activated on Sunday if, as expected, Sydney's dam levels fall below 60 per cent full. Credit:Bellinda Kontominas

The worsening drought impacts, worries about water allocation in rural areas and the desalination plant restart underscore the government's mishandling of water issues, Chris Minns, Labor's water spokesman, said.

"It's shaping up to be one of the big election battlegrounds," Mr Minns told The Sun-Herald on Saturday.