GALES and heavy rainfall will pound the NSW coast as far south as Sydney and the Illawarra region, possibly causing seawater flooding, forecasters say.
Bureau of Meteorology forecaster Julie Evans said the heaviest rainfall from ex-cyclone Oswald was likely to hit the Hunter Valley on Monday night.
The storm is expected to track south to Sydney before midnight with heavy rainfall until early on Tuesday.
A gale warning for Sydney's closed waters has been issued, with winds expected to reach 65km/h early on Tuesday morning and seas increasing up to 1.5 metres.
A road weather alert for Sydney says flooded roads and reduced visibility is expected to make driving dangerous across the city until Tuesday morning.
"We are expecting good rainfalls across Sydney overnight," Ms Evans told AAP.
"The strongest wind gusts will be on the coastal fringe.
"The winds are very much offshore, and it's a matter of how inshore they come."
Heavy surf is expected to extend down the coast to Sydney from Coffs Harbour, where waves are averaging six metres, with the biggest up to 10 metres and their effect accentuated by abnormally high tides.
"There could be seawater flooding of low-lying areas and very heavy surf, which could lead to localised damage and coastal erosion," Ms Evans said.
She said "the dangerous surf would not vanish as quickly" as the rainfall and high winds.
"It will linger on Tuesday and possibly even Wednesday," she said.
The Illawarra region is also likely to experience strong winds as the low-pressure system moves past Sydney.
"Initially, we didn't think it would extend so far south," Ms Evans said.
A severe weather warning for destructive and damaging winds, heavy rain, abnormally high tides and damaging surf is in place for the Illawarra region and could extend to the south coast.
The State Emergency Service has told people to move vehicles away from trees, secure loose items around houses and balconies and keep clear of fallen power lines.
People should not drive, ride or walk through floodwater, should keep clear of creeks and storm drains, and if trapped by flash flooding should seek refuge in the highest available place.
Ms Evans said some towns in northern NSW had received more than 900mm of rain in the past 24 hours.
People should ring triple-0 if they need rescuing.
Royal Life Saving Society chief operating officer Justin Scarr said the flooding and high seas were a concern, given there were more than 30 flood-related deaths in the 2011 floods.
A pupil-free day in NSW was also a worry, Mr Scarr said.
"We'd urge parents to sit their children down and talk about the dangers of swimming in flooded waterways or playing near waterways because there were a number of child drownings in the last floods," he told AAP.
Mr Scarr said coastal waters would be incredibly polluted and seas dangerous.
"The conditions tomorrow will not even be (safe) for experienced, surfers."
A high proportion of people who drowned in floods were older people trying to get their cars through causeways to get home, Mr Scarr said.
People should leave rescues to emergency services personnel.
"In flooded rivers, this is particularly dangerous and people shouldn't enter the water in an attempt to save someone else," Mr Scarr said.
