Sydney's $2.3 billion desalination plant is set to restart over the long weekend, as catchment levels hover just above the trigger point and inflows were revealed to have dived almost 90 per cent below average in 2018.

Dam levels were at 60.1 per cent on Friday, and at a current depletion rate of 0.4 percentage points, will shortly fall below the 60 per cent mark that will prompt WaterNSW to ask for the desal plant to be switched on for the first time since mid-2012.

Cordeaux Dam, in the Illawarra region, is now only about one-third full, among the lowest with the Great Sydney catchment. Nick Moir

WaterNSW said inflows last year reached 143 billion litres, barely above the 1944 low of 136 billion litres. Average inflows are just under 1.4 trillion litres.

Dam levels have dropped about a fifth over the past year, a faster decline than even during the severe Millennium Drought earlier this century. In 2004, for instance, inflows were 233 billion litres, WaterNSW said.

Adding to the drain on water is Sydney's growing population, as well as the long dry spell and above-average temperatures, Sydney Water said.

Total water consumption last year reached 587 billion litres – or about four times the inflows. That was 1.4 per cent higher than in 2017.

Still, that thirst was well below the record demand of 684.3 billion litres in 1991. Correcting for weather, daily water use has dropped to about 200 litres as of 2018, compared with 260 litres in 1991, Sydney Water said.

Even if the desal plant gets activated, the restart amounts to a recommissioning after so long a dormancy and following major repairs from a tornado hit in late 2015.

The operators don't expect potable water to be produced for about three or four months, and full production of 250 million litres – about one-sixth of Sydney's daily needs – will require about as long again.