Another damp weekend is in store for Sydneysiders, with the risk of heavy rainfall and even flash flooding stretching all the way down the coast from Gosford to the Victorian border as visitors flock to the city for the Mardi Gras.

Forecasters were expecting a low-pressure system to develop off the NSW Central Coast on Friday and intensify as it drifts south.

The Bureau of Meteorology on Friday issued a severe weather warning for heavy rainfall and the possibility of 24-hour rainfall totals of as much as 100 millimetres.

Affected regions include Sydney, Penrith, Parramatta, Katoomba, Gosford, Wollongong, Nowra, Bowral, Batemans Bay, Bega, Moruya Heads and Eden.