The NSW State Emergency Service has made six flood rescues after drivers became stuck in rising waters across Sydney as an unprecedented downpour saw a month's worth of rain fall in two hours.
The SES has received 200 calls for help as a line of severe thunderstorms closed in on the city before the morning commute.
Parts of the city have already recorded about 100 millimetres of rain - more than the November average - while areas in the Blue Mountains have received over 80mm.
One of the worst-hit areas was Marrickville, where three cars became stuck in floodwaters.
In Glebe, roads looked more like rivers as flash flooding left its mark near the Wentworth Park greyhound track.
Major arterials were impacted, including Parramatta Road which was closed at Concord and the Western Distributor which was closed westbound on the Anzac Bridge at Pyrmont.
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."
Ausgrid reported about 3500 power outages in both North Ryde and East Ryde, while there were also outages in the CBD.
Sydney Airport has closed two runways due to the storm and a spokesperson said delays and cancellations are likely today.
Only one runway is in operation, and customers are being advised to check with their airlines before arriving at the airport.
'Dangerous' thunderstorms cut power
Bureau of Meteorology forecaster Rob Taggart said in the space of one hour this morning, almost 70mm of rain fell at west Pennant Hills and 61mm at Mosman in just half an hour.
"That's phenomenal rainfall," he said.
But by 8am, the thunderstorms had eased and moved offshore.
"At the moment we're looking at a period of moderate rainfall, but rainfall could increase again during the morning and early afternoon," Mr Taggart said.
"It's no surprise to hear of flash flooding all over the place."
Forecasters previously predicted up to 200mm of rain was expected to be dumped on parts of Sydney due to a deepening low over central NSW moving towards the coast.
The downpour wreaked havoc on the morning commute, with trains temporarily halted between Campsie and Sydenham and ferries suspended between Parramatta and Olympic Park due to flooding.
The light rail services between Dulwich Hill and Central were also suspended.
Transport for NSW Coordinator General Marg Prendergast said buses were replacing suspended services.
"We are struggling to get buses because they're stuck out there on the road network," he said.
"What we've got is traffic signals out, and we've got lots of localised flooding."
An updated Bureau of Meteorology warning for severe weather, issued early this morning, predicted "intense rain" and "vigourous winds".
The warning extended to parts of the mid-north coast, Hunter and Central Tablelands.