Electricity crews are scrambling to restore power to more than 40,000 homes and business hit by stormy weather in Sydney's south on Saturday.

After a sunny start to the day, the expected storms rolled in, with the Bureau of Meteorology issuing a severe thunderstorm warning for large parts of north-western Sydney.

Spectacular lightning and some hail was reported across chunks of western Sydney, and electricity company Ausgrid has reported that emergency crews are working to restore power to 40,000 residents in the city's south from Caringbah to Waterfall after the storm cut power to high voltage cables in the area.

Fiona Morris Storm clouds at Rooty Hill before the massive storm hit on Saturday.

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Endeavour Energy also reported that power has been cut to more than 18,000 homes and businesses across western Sydney due to the storm late this afternoon. That includes 6000 homes between Merrylands and Guildford, 3000 in the Hills district and 1600 in Liverpool.

NICK MOIR A massive storm hammer north-western Sydney earlier in the week.

A spokesman for Endeavour said emergency crews "are being hampered in patrolling these areas to make safe and commence the restoration process by ongoing lightning and heavy rain".

"Until these patrols are undertaken it is not possible to give an estimate of the time it will take to restore supply and we would appreciate affected customers being patient," he said.

Ausgrid crews were also working to restore power to about 1400 residents in Wahroonga and Waitara.

Weatherzone meteorologist Sam Terry said a severe thunderstorm had led to "significant rainfalls" in parts of western Sydney, including more than 15 millimetres in a 10 minute period near Badgerys Creek.

"That's more than enough to lead to flash flooding," he said.

Terry said the worst of the rainfall hit in places like Penrith, which saw 40 millimetres, and Toongabbie, which saw about 3 millimetres.

A "severe" gust of wind of 98 kilometres per hour was also recorded at Badgerys Creek, while Bankstown saw gusts of about 80 kilometres.

"You're likely to see some branches ripping off trees or loose roof tiles being moved around," he said.

The State Emergency Services has received about 300 calls for assistance in the last few hours, as well as six flood rescues, with spokesman Phil Campbell saying some callers were being greeted by a busy tone because the call centre was being "maxed out".

"Such is the volume of requests for help we're getting," he said.

It is the second storm Sydney has suffered through in two days, with Campbell saying the majority of the focus had again been in the west.

"We had 800 calls for assistance last night, so you could say it's 1100 for the weekend," he said.

The Bureau of Meteorology issued and updated warning just after 6pm saying that further storms were still possible.

Severe thunderstorm warnings are also covering parts of the south-west including Camden, Oran Park and Picton.