Sydneysiders will have their first water restrictions imposed in almost a decade to help stem a rapid decline in the city's reservoirs amid the state's ongoing drought.

The Berejiklian government decided to bring forward the level 1 curbs to June 1, or about two months earlier than would be triggered under the Metropolitan Water Plan.

The formal trigger for such restrictions is when dam levels hit 50 per cent. On Tuesday, they were at 53.5 per cent - and losing 0.5 percentage points per week - down almost half in two years, according to WaterNSW.

Cordeaux Reservoir was at 40 per cent full as of May 28, compared with an average of 53.5 per cent capacity across Sydney's dams. Credit:Nick Moir

Sydney Water have said the city's dams have fallen faster over the past two years than during the Millennium Drought, with inflows at levels not seen since the 1940s.