The NSW State Emergency Service (SES) is warning people to stay indoors as severe weather strikes Sydney, the Illawarra and the Blue Mountains.

Sorry, this video has expired Cars stranded in floodwaters at Marrickville in Sydney ( Jessica Kidd )

SES spokesman Phil Campbell said there had been 620 calls for help and volunteers had performed 40 flood rescues.

"The flood rescues that we're receiving are in the Illawarra, as well as in the south-eastern suburbs of Sydney around the Sutherland Shire," Mr Campbell said.

"We are asking people to avoid any non-essential travel and to stay well clear of any flood waters they might encounter."

The Bureau of Meteorology warned dangerous storms detected on its radar were likely to affect Sydney Airport, Bankstown, Maroubra, then the CBD, Sydney Olympic Park and the Sydney Harbour Bridge.

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About 30,000 homes were without power and three major substations were out in the Blue Mountains.

Peter Payne from Endeavour Energy said major power stations in the Blue Mountains were affected by a snow storm.

"A snow storm has interrupted the main transmission supply into three of our major zone substations being at Blackheath, Katoomba and Wentworth Falls. Those zone substations supply about 15,000 customers," he said.

Thunderstorms caused delays at Sydney Airport because ground staff could not go out onto the tarmac to unload planes.

Wind gusts of 160 kilometres per hour have been recorded south of Sydney.

The SES said 60 passengers who had been stuck on a train between Kingsgrove and Bexley North were safely removed.

Other calls for assistance were for damage to property and trees and downed power lines.

In the Blue Mountains, the SES said there were reports of snow falls of up to 20 centimetres, causing road closures.

Lithgow, Katoomba, Oberon, Mount Boyce, Yetholme and Bell were all affected.

The Great Western Highway and the Bells Line of Road were closed between Katoomba and Bathurst.

Drivers were being warned to take care.