The NSW State Emergency Service (SES) says it is receiving a call every minute as severe weather strikes Sydney, the Illawarra and the Blue Mountains.

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

Flash flooding in Sydney and the Illawarra led to volunteers rescuing people trapped in cars.

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Energy provider Ausgrid said power outages were affecting 5,500 homes in parts of Bexley, Bankstown and Blakehurst as the storm passed through Sydney’s south.

Endeavour Energy said it was responding to around 4,000 customers without power.

The areas affected include the northern shore of Jervis Bay on the south coast, the southern highlands and Wollongong.

Thunderstorms were causing delays at Sydney airport because ground staff could not go out onto the tarmac to unload planes.

Meanwhile, snow fell in the Blue Mountains, causing road closures.

A radar image showing storm activity over Sydney and the Blue Mountains. Photo: Bureau of Meteorology

Phil Campbell from the SES said people should avoid going outside.

“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,” he said.

“These are a result of the flash flooding that we’re currently seeing in that area.

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

Up to 5cm of snow in Blue Mountains

In the Blue Mountains, the SES said there were reports of snow falls of up to five centimetres at Lithgow, and snow at Katoomba, Oberon, Mount Boyce and Yetholme.

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

Drivers are warned to take care.