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.

Sustained southerly winds of up to 40km/h, with gusts of between 50 and 60km/h, are also likely on Saturday.

A top of 26 degrees is forecast in the city and 27 degrees in Sydney's west on both Friday and Saturday.

It comes after several homes were flooded overnight in Londonderry, near Penrith, where State Emergency Service (SES) crews spent the night filling 2000 sandbags and protecting properties.

Two houses and two granny flats were flooded by about 40 centimetres of water, the SES reported.

SES spokesman Phil Campbell said the flooding was a "fairly isolated incident" and was due to a blocked drain in the area.

Weatherzone meteorologist Joel Pippard said the low-pressure system was expected to form off the coast late on Friday evening.

"That will direct a lot of the heavy rain towards the South Coast," he said.

Rainfall totals of more than 100 millimetres in a 24-hour period are possible in Wollongong, Nowra, Bowral, Batemans Bay, Eden and Moruya Heads, the Bureau of Meteorology warned.

"For Sydney, we should receive a moderate amount of rain, and also Sydney looks to have the most powerful winds from this system," Mr Pippard said.

Those conditions are likely to put a dampener on the Sydney gay and lesbian Mardi Gras parade on Saturday night, although the heaviest rain is likely to fall earlier on Saturday, he said.

"On Saturday evening, the rain should be lighter, but it really depends on the movement of that system," he said.

"The system is expected to move south and swing south-easterly at the moment, so it looks like the heaviest rainfall will be Saturday morning in Sydney."

 Mr Pippard said the rain should ease on Sunday, depending on the movement of the system.

The bureau is forecasting only a few millimetres of rain in Sydney and in Wollongong on Sunday.

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