Power is out to thousands of homes and the extent of damage caused by giant hailstones is unclear after a series of thunderstorm cells lashed Sydney.
Very dangerous thunderstorms were detected near Blacktown and Riverstone, the Bureau of Meteorology said on Thursday evening.
The storms moved east with severe thunderstorms hitting the city and eastern suburbs by nightfall.
Images from the city's west show golf ball-sized hailstones pummelling cars while almost 20,000 lightning strikes were recorded across the greater Sydney area.
The State Emergency Service warned residents to move cars under cover or away from trees, secure loose items around homes and keep at least eight metres away from fallen power lines.
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But the downpour triggered flash flooding, power outages and more than 350 calls to the rescue service.
Roads, trains and even flights were delayed and diverted away from Sydney's airports.
"NSW SES reports hundreds of calls for assistance while trains, roads and flights have been impacted," BoM NSW wrote on Twitter.
The conditions were a result of fairly warm and humid air interacting with a low-pressure trough lingering near the state's coast, a Bureau spokesman told AAP.
The big wet could last until early or mid next week.
Friday is forecast to bring showers and thunderstorms to the state's east possibly extending to the northwestern slopes.
Showers are forecast on Saturday extending to northern inland areas while on Sunday showers will likely reach the far northern inland with a chance of thunderstorms in the north.
