Sydneysiders have been told to reconsider their Wednesday morning commutes, with up to 200 millimetres of rain expected to be dumped on parts of the city, causing flash flooding and chaos on roads.

Key points: Damaging winds, averaging 60-70 kilometres per hour, are likely along the coast

Damaging winds, averaging 60-70 kilometres per hour, are likely along the coast Flash flooding is predicted for Sydney's north and south

Flash flooding is predicted for Sydney's north and south The most intense rainfall is expected on Wednesday, according to BOM, with some areas set to receive as much as 200mm

A deepening low over central NSW has been moving towards the coast and has triggered a severe weather warning for damaging winds and heavy rainfall in Sydney, Illawarra and parts of the mid-north coast, the Hunter and Central Tablelands districts, with flooding expected in several rivers in coastal areas.

At 10.13pm (AEDT) on Tuesday, the Bureau of Meteorology (BOM) warned intense rain and vigorous winds would affect the greater Sydney area and parts of the Illawarra on Wednesday.

The BOM's NSW severe weather manager, Simon Louis, said the system was expected to cross the coast somewhere between Sydney and Wollongong, with falls of up to 200mm expected.

That is comfortably more than the November average (83.6mm) and December average (77.5mm) at Observatory Hill combined.

"For a lot of the area we're expecting that we'll have 50 to 100mm," he said.

"But those places that do end up under that most intense band of rainfall could get well up over that, so potentially up around the 150 to 200mm mark.

"The falls are most likely to become intense tomorrow morning, potentially sometime around the morning commute period and will continue through most of [Wednesday] before easing in the afternoon or late evening."

Heavy rainfall and damaging winds are predicted for Sydney and Illawarra on Wednesday. ( Bureau of Meteorology )

State Emergency Service assistant commissioner Scott Hanckel urged people to reconsider their travel plans.

"If you're just doing your normal commute in it's just going to be problematic, businesses need to expect people to be late," he said.

"Any pre-planning (commuters) can do today around their arrangements … is going to be of great value to you."

NSW Police Chief Inspector Phillip Brooks said motorcycle riders, cyclists and pedestrians were most at risk on Wednesday morning.

"We're asking all road users, particularly those that are considered vulnerable … to perhaps reconsider their need to be on the roads, reschedule their day and then continue their journey on safely after the rain has passed," he said.

Strong and damaging winds are expected to develop across a wide stretch of the NSW coast once the low moves offshore on Wednesday afternoon.

Heavy rain and flash flooding is likely to impact parts of the Illawarra, Sydney, central coast and Blue Mountains, but is expected to ease later in the day as the low-pressure system moves offshore.

Minor to moderate flood warnings are also in place for the Nepean, Hawkesbury, Cooks and Georges Rivers.

"There could be some short-term isolation for some of the communities in the Nepean area," Assistant Commissioner Hanckel said.

"At the moderate level some individual properties might have some water threatening, depends where the heaviest rain falls."

People have also been urged to avoid swimming, rock fishing and boating along the Sydney coast and Illawarra coast due to the hazardous surf and swell conditions.