The storm has eased, but the Bureau of Meterology is still warning that Sydney could be hit by another storm on Saturday night.
Severe thunderstorms battered the city on Saturday afternoon, causing power outages and flash flooding in parts of southern and western Sydney.
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'Very dangerous' storm batters Sydney
A thunderstorm brings down trees in Sydney's north-west as energy suppliers scramble to restore power to more than 50,000 affected homes.
After a sunny start to the day, heavy rainfall and thunder rolled into Sydney for the second time in two days at about 4pm, with the Bureau issuing a severe thunderstorm warnings for large parts of Sydney.
Weatherzone meterologist 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.
Mr Terry said the worst of the rainfall hit in places like Penrith, which saw 40 millimetres, and Toongabbie, which saw about 30 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.
After an earlier outage, Ausgrid managed 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.
But power still remained out in some areas, with Endeavour Energy reporting electricity had 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.
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.
It is the second storm Sydney has suffered through in two days, with Mr 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," hesaid.
The Bureau of Meterology issued and updated warning just after 6pm saying that further storms were still possible.
At about 4.20pm, the Bureau warned that "very dangerous thunderstorms" have been detected in parts of western Sydney including Richmond, Wilberforce and Windsor. It said the storms are forecast to hit parts of the north-west such as Dural.
It also issued severe thunderstorm warnings covering parts of the south-west including Camden, Oran Park and Picton.
The Bureau advised that severe thunderstorms detected in the areas surrounding the Blue Mountains were moving south, and would hit Sydney's south-western fringe, including Appin, south of Campbelltown, by 4pm.
Temperatures reached highs of between 29 and 31 in Sydney's west on Saturday, with conditions slightly cooler toward the city centre.
According to Endeavour Energy, 16,000 homes experienced power outages in Greater Western Sydney on Friday, and a further 20,000 serviced by Ausgrid across the state.