Ex-tropical cyclone Oswald is expected to reach the Illawarra about dawn today before moving out to sea.
A severe weather warning for destructive and damaging winds, heavy rain, abnormally high tides and damaging surf remains in place for the Illawarra and could extend to the South Coast as the low-pressure system moves south.
Bureau of Meteorology forecaster Katarina Kovacevic said heavy rainfall of 60 to 80 millimetres was expected to fall in the Illawarra overnight last night, with winds of up to 65km/h.
State Emergency Service (SES) crews remained on standby throughout the region last night. There had been few requests for help by 7pm. But NSW SES spokeswoman Jacqueline Sergeant said they were bracing for an onslaught of calls because of the predicted bad weather.
"We are expecting heavy rain and strong winds to hit the Illawarra between midnight and 6am," she said.
"So we are expecting many requests for assistance during that time and throughout Tuesday [today] with concerns due to the fact that most people will be returning to work in less than favourable conditions."
Ms Sergeant advised intending drivers to check Live Traffic NSW (livetraffic.com) for road closures in their area from flash flooding.
She said SES crews were expecting to need to rescue some people, as areas of the Illawarra were flood-prone.
"People should not drive, ride or walk through floodwaters and should keep clear of creeks and storm drains," she said.
She suggested securing or putting away loose items in the yard, such as trampolines, and moving vehicles under cover.
Illawarra holidaymakers heading home yesterday experienced heavy traffic conditions. There was an accident at the intersection of Lake Entrance Road and the Princes Highway at Oak Flats about 1.30pm.
About 6.30pm one northbound lane was closed on the Princes Highway at Russell Vale due to localised flooding, and one southbound lane was closed on Mount Ousley Road.
Meanwhile, trains were suspended last night on the Port Kembla branch line between Wollongong and Port Kembla due to flooding at Port Kembla.
Buses were replacing trains between Wollongong and Port Kembla and minor delays were anticipated, so commuters should allow extra travel time and check services today.
Illawarra beaches are expected to remain closed due to abnormally high tides.
Beaches will be dangerous and people are advised to stay well away from the surf and surf-exposed areas.
Ms Kovacevic said the strong winds and heavy rain would gradually ease throughout today.
"Heavy rain should taper off from the afternoon and only light shower activity is expected for Wednesday and into Thursday morning - we could even see a sunny afternoon on Thursday," she said.
"Strong north-easterly winds will ease late Tuesday morning before we see some strong southerly winds picking up in the afternoon, and then an easing in conditions."