Sydney's road and transport network is in chaos and hundreds of people have been rescued or forced to flee their homes after the city suffered the sort of weather seen once every century.
"We've had a hell of a rain event," NSW Roads Minister Duncan Gay said.
"It's that one in a hundred year event that you hear of."
Flooding has closed dozens of roads and some rail lines and train stations, while ferries have been cancelled and there are flight delays at Sydney airport.
Authorities have been called out to more than a thousand incidents on the roads and locals in Sydney's inner west were given evacuation orders.
Only the roofs of cars were visible on some streets and there are fears that hundreds of boats on Sydney harbour could sink.
"We've got boats filling up with water from all this heavy downpour," NSW Maritime spokesman Neil Patchett told ABC Radio.
About 119 millimetres of rain fell on parts of the CBD this morning, with the Observatory Hill weather station recording its highest daily rainfall total since 2007.
In the city's west, 146mm fell on Merrylands.
The Cooks River, in Sydney's south, flooded when it hit 1.5 metres about 9.30am (AEDT) at Tempe Bridge.
"There were some properties around that area of Tempe that might have been subject to flash flooding," SES spokesman Andrew Richards said.
Mr Richards said the SES expected to be moving its resources to areas south of Sydney as the heavy rainfall moves south.
Electricity provider AusGrid said power had been lost to 2000 homes and businesses at 60 different sites, including Chippendale, Brighton-Le-Sands and Lidcombe.
More than 20 people were evacuated from the Scalabrini Retirement Village in Austral, southwest Sydney, after the Bonds Creek flooded.
Residents of 20 homes in three streets at Marrickville were also told to leave because of severe flash flooding.
A water spout terrorised a suburban street in Kingsford and there are reports of almost 200 students being evacuated from a primary school on the outskirts of Sydney.
Motorists were rescued from cars caught in flooding at Rossmore, Austral and Campbelltown.
Mr Gay said roads had been closed across the city "that haven't been closed before".
"Our Traffic Management Centre indicates it is the worst day we've in the 15 years it has been operating," he told reporters in Sydney.
"It's a tough day out there. It is a weather the event the like of which many of us have never seen before."
Mr Gay urged people to stay home if they could, saying the flooding had even stopped some state MPs from attending parliament on a sitting day.
"We certainly recommend if your travel is difficult, to stay at home," he said.
"I'm sure employers would be understanding in this situation."
Mr Gay dismissed suggestions the widespread flooding had shown a need to improve road infrastructure across Sydney.
"The roads are flooding because we've had a hell of a rain event - it is as simple as that," he said.
"And when you get that situation, it certainly tests all your maintenance, and all your stormwater drains."
