Commuters are being warned to avoid driving or riding in and around Sydney and other parts of NSW on Wednesday, with the state's biggest cities to be hit with a month's worth of rain in less than 24 hours.
Up to 100 millimetres could fall across Sydney, the Hunter and Illawarra, the Bureau of Meteorology says. Some places could top 200mm.
The mean November rainfall in Sydney is 84mm.
While it's unclear where it will be most intense, the weather system is expected to reach the NSW coast during the Wednesday morning peak.
"We're asking all road users to perhaps reconsider the need to be on the road through what will be a severe rain event," NSW Police Chief Inspector Phillip Brooks told reporters on Tuesday.
"Drivers, riders, cyclists and pedestrians need to make sure they are safe on our roads."
NSW State Emergency Service Assistant Commissioner Scott Hanckel said parents should think about alternatives for the school drop-off and pick-up and advised businesses to expect workers to arrive late.
"It's a great day to work from home - if that's suitable," he told AAP.
Mr Hanckel stressed drivers not drive into flood waters - with that being the overwhelming major cause of flood deaths.
A severe weather warning for heavy rain, possible flash flooding and damaging winds has been issued for Sydney, the Illawarra and parts of the Hunter and Central Coast regions.
Severe Weather Manager Simon Louis said it could lead to roads awash with water and rapidly rising creeks.
He said "one good thing" about the system was that it would pass quickly.
But he warned that once offshore in the evening, damaging southerly winds are likely to buffet the coast and whip up hazardous surf until Friday morning.
