Sydneysiders are being warned to brace for more "fast and furious" storms on Friday, as emergency crews continue the clean-up from wild weather overnight.
The NSW State Emergency Service has received more than 1000 calls for help since Thursday evening, as hail, heavy rain and ferocious winds battered Sydney and its surrounds.
The northern beaches, western suburbs and Illawarra were hit hardest by the cluster of very dangerous storms, which formed over the Southern Highlands.
Mona Vale recorded 120mm of rain in two hours, while multiple areas were impacted by hail as large as golf balls - including Kellyville and Blacktown, according to the Bureau of Meteorology.
Almost 20,000 lightning strikes were recorded across the greater Sydney area, with more than 25,000 homes and businesses losing power.
The SES responded to about 600 jobs, mostly for fallen trees and damaged and leaking roofs, as well as three flood rescues involving cars.
Spokeswoman Natasha Wainberg said more bad weather was predicted for Friday afternoon.
"A very similar, really nasty short burst of fast and furious weather," she told AAP.
"We are stressing to people - with the likelihood of this rain pattern continuing and these bursts - please stay off the roads, adjust your travel pattern, and please do no drive through flooded roadways."
People are being urged to secure any loose items around their homes, and plan their travel around storm warnings.
Bureau of Meteorology forecaster Jordan Notara said showers were predicted across Sydney on Friday, with a chance of storm activity in the afternoon.
Heavy rainfall, particularly in the city's coastal areas, is also forecast for Saturday and Sunday.
Sydney Airport had large queues of passengers on Friday morning, with airlines warning of knock-on delays following more than 40 flight cancellations on Thursday.
Vimal Kumar, who was flying to Melbourne, tweeted that stranded passengers were sleeping on baggage carousels on Thursday night.
At least 15 domestic flights in and out of Sydney were delayed and 13 cancelled on Friday morning.