Sydney is set for a wet Sunday, with heavy rainfall intensifying in the afternoon, as an east coast low brings a severe weather warning to the Illawarra and south coast.

A clear Saturday evening will give way to rain before dawn, which will steadily intensify into the afternoon as the low forms off the mid-NSW coast. 

Sunday is expected to be Sydney's wettest of the coming days since the last winter storm two weeks ago and emergency crews expect to be on standby until at least Monday when the weather is likely to clear. 

After months of little rain, storm cells lashed the East Coast in early June. Photo: James Alcock/Fairfax Media

But the downpour will be only a fraction of 100-200mm recorded during that last storm to drench the city, with just 15-35mm of rainfall predicted before Monday, the Bureau of Meteorology says. 

Some parts of Sydney may receive more rainfall as heavier bands within the larger rain band make their way across south along the coast and off to the east. 

"There will be some dry patches throughout Sunday, but generally it'll be a wet one", said Bureau of Meteorology forecaster Gabrielle Woodhouse. 

View of the city from Berrys Bay as rain clouds roll in. Photo: Sarah Keayes

"The rain will start easing on Monday, with between two and three millimetres predicted before clearing on Tuesday," Ms Woodhouse said. 

Northerly winds will start to intensifying on Sunday, with the bureau issuing a strong wind warning for the Sydney, Illawarra and Bateman coasts on Sunday.

The low is expected to form near the Illawarra coast on Sunday evening before deepening and moving south early Monday. Heavy rain in excess of 100mm, possibly 150mm may lead to flash flooding in the area.

Damaging winds averaging 60 to 65 km/h with peak gusts in excess of 90 km/h are possible along the coastal fringe from the early hours of Monday.

A gale warning is in place for the Byron, Coffs, Macquarie and Hunter coasts. 

King tides no longer threaten the coast, lowering the prospects of coastal erosion though some areas of the coastline are still open to further damage, Ms Woodhouse said. 

The bureau also issued a flood watch on Friday afternoon, identifying a greater than 70 per cent chance of flooding in nine river valleys in NSW from Sunday onwards.

Most of the nine are expected to have minor flooding, although the Nepean-Hawkesbury is among three catchments that may get moderate flooding.