Several homes in Sydney's west have been flooded overnight as parts of NSW brace for torrential rain and potential flash flooding this weekend.

The Bureau of Meteorology has issued a severe weather warning for heavy rainfall in the Illawarra, South Coast and parts of the Southern Tablelands and Snowy Mountains forecast districts, as a low-pressure system develops off the NSW coast late on Friday.

More than 100 millimetres of rain could drench those regions in 24 hours, the bureau has warned.

Sydney won't escape the deluge, with moderate falls expected to deliver up to 35mm of rain for the city on Friday and 25 millimetres on Saturday, with coastal falls of up to 40mm possible on Saturday.