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.

The weather at Sydney Airport on Friday evening caused at least six flights to be cancelled, including flights from the Gold Coast and Melbourne, while others were delayed.

On social media, the airport warned more rain was predicted for the weekend, advising travellers to check its website for information on arrivals and departures.

Minor flooding was predicted for the Moruya, Bega and St Georges Basin river valleys in the state's south, with the last of the three coinciding with a high tide at Sussex Inlet.

For Sydney, Saturday morning sport may be affected before conditions clear to a few showers by lunchtime, although Mardi Gras marchers and watchers may still need a brolly when the parade kicks off on Saturday night.

Thunderstorms are a risk in the Sydney Basin on Saturday, especially in the morning, Gabrielle Woodhouse, a duty forecaster at the bureau, said.

Mopping up is likely to continue in parts of Londonderry near Penrith, where several houses were flooded overnight into Friday.

Current forecasts indicate rainfall totals about 30-50 mm for Sydney on Saturday and a couple more showers on Sunday. 

The run of partly cloudy, showery days will extend well into the next week until a high-pressure ridge arrives, setting up a gradually warming trend, Graeme Brittain, a meteorologist from Weatherzone, said. 

That improving outlook means those who have lately given up on weekend sport or beach trips should have something to look forward to.

With Megan Levy

Weatherzone is owned by Fairfax Media, publisher of this website.