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.

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.