The Berejiklian government is facing calls to accelerate the shift to water restrictions to slow the decline in Sydney's dam levels rather than flicking the switch to expand the city's $2.3 billion desalination plant.

Storages of water have almost halved in two years amid the lowest inflows since the 1940s, Sydney Water said.

Reservoir levels continue to slide about 0.4 per cent a week - dipping to 53.7 per cent full as of Saturday - even as the desal plant ramps up faster than scheduled. The facility is running at just below half its 250 million litres daily capacity and is expected to be operating at full tilt by August, a spokesman for the plant said.

Water Minister Melinda Pavey is expected to outline a series of water options to state cabinet on Monday.

One issue is likely to be whether the government should move immediately to impose level 1 water restrictions rather than wait for the formal trigger if dams sink below 50 per cent full. That mark is also supposed to prompt preliminary planning for a doubling of the desal plant's capacity to supply 30 per cent of Sydney's use.