Coastal NSW can expect another day of heavy rain, with forecasters warning of flash floods and thunderstorms.

A severe weather warning is in place on Wednesday for coastal areas of Sydney, the Hunter Valley, the state's south coast, Illawarra and mid-north coast, with heavy rains not expected to ease for at least 24 hours.

The weather has already caused traffic chaos for some motorists heading to work on Wednesday.

Localised flooding has closed Oxford Falls Road in Sydney's north while similar sloshy conditions have banked up traffic at Bondi Junction in the city's east.

The Central Coast was lashed with particular force, with 102mm of rain falling around Wyong in the past day.

Suburbs in Sydney's north prone to heavy downpours were also hit, with Hornsby taking 72mm and Wahroonga copping 70mm.

Bureau of Meteorology senior forecaster Jane Golding said residents in coastal areas looking forward to drying off on Wednesday should prepare for another wet and wild 24 hours.

"The same areas (hit by recent rains) are under threat of some heavy showers," she told AAP.

"With that, we tend to see some localised road flooding and there's also the threat of flash flooding."

The low pressure trough off the NSW coast isn't expected to bring a reprieve until late Thursday or early Friday.

A flood watch is currently in place for the Orara, Bellinger and Lower Hunter River valleys.