A complex low-pressure system is slowly deepening over the Tasman Sea, bringing belts of rain over Sydney. Wind gusts are starting to pick up, particularly on the coastal strip.

"Winds are going to be peaking in the middle of the day," Rob Sharpe, a meteorologist with Weatherzone, said.

Cronulla and Little Bay have picked up the most rain from the event so far, with more than 20mm, while Point Perpendicular on the Illawarra coast collected about 60mm, he said.

The Bureau of Meteorology is predicting rainfall of as much as 40mm for the city on Friday, and has issued a severe weather warning for damaging winds for the Sydney Metropolitan, Hunter and Illawarra districts.

"The coast is going to be seeing the worst of all of the conditions: the heaviest rain, the strongest winds and also the dangerous surf," Mr Sharpe said.

Since midnight, the strongest winds have reached 115 km/h at Wattamolla south of Sydney just after 10.20am while Sydney Airport and Sydney Harbour rocorded 87km/h gusts in the early afternoon, the bureau said.

Weatherzone: Sydney radar

The mercury reached 17.7 degrees by 1pm although the strong winds meant perceived temperatures were several degrees cooler.

The rain is likely to tend to showers in the afternoon but persist into the evening and Saturday morning, he said.

"For anyone heading to Vivid Sydney, it's going to be a cold, wintry night, with the chance of a few showers coming through," Mr Sharpe said.

Saturday will most likely be another cool day with a top of only 17 degrees. The mercury, though, should start to climb back to a relatively balmy 24 degrees by Thursday, with little change of rain from Sunday to then.

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