A building in Sydney's north has had its walls sucked off by a "mini cyclone" as a thunderstorm left widespread damage and sparked flash flooding emergencies around the city on Wednesday.

Key points: Parts of Sydney received more than 100mm of rain on Wednesday morning

Parts of Sydney received more than 100mm of rain on Wednesday morning The NSW State Emergency Service made more than a dozen rescues

The NSW State Emergency Service made more than a dozen rescues Stormy conditions were expected north of Gosford overnight

The windows of a Chatswood office building were blown out and tossed aside by battering winds which reached 70 kilometres per hour in some areas.

"I started to go upstairs to see if there was any damage, and next minute there was an almighty bang and it ripped panels off the roof in the warehouse," building manager Greg Transell said.

"It was amazing actually because you can see … how it's tried to actually suck, it's sucked some of the doors the reverse way out through the frames and actually dislodged the frames and all the rubber lining that holds the windows.

"All that's come loose and it's tried to suck all the glasswork out."

The Ausgrid network at Chatswood was damaged in the deluge. (Twitter: Ausgrid)

Mr Transell said the winds had "enormous suction power" while another person who saw the destruction unfold described it as a "mini cyclone".

"It really was quite scary actually because you feel like you're actually getting sucked out," Mr Transell said.

The building houses a medical supplies company, and many workers had not yet arrived at the office when the windows were blown off around 6.00am AEDT.

The biggest November downpour in Sydney in more than three decades wreaked havoc and the State Emergency Service (SES) had to respond to 14 car rescues from floodwaters.

A female police officer was taken to Royal North Shore Hospital in a serious condition after a tree fell on her while she was trying to help trapped motorists.

Sorry, this video has expired Wild weather continues to lash Sydney

NSW Police Assistant Commissioner Michael Corboy scolded some "idiotic" drivers.

"We are really worried about people trying to cross flooded streets or flooded streams in this weather. [And] we've already seen idiotic driving today," he said.

"It is extremely dangerous practice, particularly in smaller cars, and we've seen people get washed away and killed before on many occasions, both in NSW and throughout Australia."

Assistant Commissioner Corboy said a 14-year-old passenger died in a two-car collision in Thornleigh, in Sydney's north-west.

The SES received more than 1,600 calls for help and at around 10.00pm Ausgrid was still working to restore power to over 3,000 customers across Sydney and the Central Coast after around 8,000 were left in the dark after the height of the storms.

A severe thunderstorm warning for Sydney, Wollongong and Newcastle was cancelled in the evening but a severe weather warning currently remains in place for rainfall and wind.

However the BOM said conditions were generally easing as the storm made its way offshore.

Sorry, this video has expired Sydneysiders wake up to heavy rain

Record rainfall hits Sydney

Marrickville and Glebe were inundated, while 111 millimetres of rain was dumped on Mosman — including 61mm in half an hour on Wednesday morning.

Sydney's November average is 83.6mm, and the Bureau of Meteorology (BOM) confirmed it was the wettest November day since 1984.

In the Inner West, roads looked more like rivers as flash flooding left its mark near the Wentworth Park greyhound track.

Some parts of the Blue Mountains received more than 80mm.

Supermarket workers at Town Hall work desperately to stop water flooding their shop. (Twitter: @jameslemon)

SES spokeswoman Terri Langendam said rescues had also been performed at West Pymble, Silverwater and Macquarie Park.

"They're just basically people that are driving through deep water thinking they can get through," she said.

"They're getting inundated with water, their vehicle's shut down and they're stuck in their car.

"Floodwater can be difficult to judge, turning around and delaying travel is always the best option."

Two police officers were injured about 6.50am when they were called to Pittwater Road, North Ryde.

The officers were assisting when a tree fell on them, breaking a female officer's leg and leaving the man with minor injuries.

At one stage, Ausgrid was working to restore power to more than 8,000 customers.

Flash flooding ran through James Gleeson's north Sydney home. (Facebook: James Gleeson)

Sydney Airport closed two runways due to the storm and a spokesperson said more than 130 flights had been cancelled or delayed.

BOM forecaster Rob Taggart said in the space of one hour on Wednesday morning, almost 70mm of rain fell at west Pennant Hills and 61mm at Mosman in just half an hour.

Some ferry, train and light rail services were suspended amid the deluge.