One person has died, two police officers have been injured and almost 100 flights cancelled as Sydney copped a month's worth of rain within hours.
A series of thunderstorms battered the coast from Newcastle down to Wollongong on Wednesday, wreaking havoc on the morning commute.
A person was killed in a crash on The Esplanade in Thornleigh, on the city's upper north shore, while a tree fell on two police officers who were trying to help drivers trapped on a flooded road in North Ryde.
A probationary constable suffered a suspected broken leg and has been taken to hospital in a serious condition, while her partner suffered a possible concussion and minor injures.
More than 105mm had fallen at Observatory Hill in Sydney's CBD by 9am - eclipsing the November average of 83.8mm.
Mosman recorded 118mm to 9am while Chatswood had 105mm, including 66mm within one hour.
Shoalhaven's Porters Creek Dam has recorded the state's highest total so far, copping 152mm to 9am.
Sydney Airport was reduced to a single runway during the morning, as it was hit by strong winds and 39mm of rain.
About 90 domestic flights and four international flights have been cancelled due to the weather.
The NSW SES has had about 500 calls for help and rescued 12 people from floodwaters, mainly in the city's north.
"In most cases, people have just driven into flood waters, followed other people into flood waters, their cars have stopped and we've had to go and help them out," Greg Murphy told Sydney's 2GB radio.
Police are urging people take care in the "horrendous weather conditions".
"We can't stress enough that motorists should never attempt to drive through floodwaters or cross flooded causeways," Assistant Commissioner Michael Corboy said in a statement.
Flooding has also forced the cancellation of trains between Sydenham and Campsie and light rail services, while ferries between Parramatta and Sydney Olympic Park have been cancelled.
Dozens of roads across the city have been shut due to flash flooding.
The Woolworths supermarket at Sydney's Town Hall was forced to close after water began pouring through the walls and roof.
More than 5000 properties are without power across the city, including the CBD and northern suburbs, with lightning strikes and strong winds damaging lines.
AusGrid said the torrential downpour flooded an underground substation at Wynyard.
A severe thunderstorm warning is still current for the Sydney metro region and parts of the Hunter, Illawarra, South Coast and Central Tablelands.
The heavy rain isn't expected to ease until early Wednesday evening, but damaging winds could hang around until Thursday.
