Forecasters for the Bureau of Meteorology said the immediate threat of severe weather has passed, but the situation will continue to be monitored and further warnings will be issued if necessary.

The revised warning came after just more than 48 millimetres of rain fell over Williamtown in less than an hour around 8am, and damaging winds, averaging 60 to 70 km/h with peak gusts above 90 km/h lashed the coastal fringe.

Flash flooding hit on Monday morning and continued throughout the day, including in Newcastle's city centre where the intersection of King and Steel streets went underwater around 9am.

The RAAF Base Williamtown and Newcastle Airport airfield is also closed due to flooding.

The limited commercial flight schedule has been postponed and passengers booked on flights should contact their airline directly for more details.

SES crews were called to four flood water rescues in just 24 hours after drivers entered floodwaters in their vehicles in separated incidents across the region.

In one early morning rescue, SES crews and emergency services were called to Louisiana Road, Hamlyn Terrace.

Crews arrived to find a small sedan stuck in the fast-moving floodwaters across the roadway with one person still in the vehicle. Rescue crews assisted the vehicle to safety, and the driver thankfully suffered no injuries.

"An important reminder to please not drive through floodwaters at all," a Central Coast SES statement said.

A hazardous surf warning remains in place along the NSW coast, including the Hunter's beaches, as NSW police advise residents to stay out of the water and to avoid walking near surf-exposed areas.

Rock fishers should avoid coastal rock platforms exposed to the ocean and seek a safe location that is sheltered from the surf, police said.

Boaters planning to cross shallow water and ocean bars should consider changing or delaying their voyage.