Drivers are continuing to ignore warnings from emergency services not to drive through floodwater, resulting in 21 rescues in two days along the NSW coast.

The SES says most of the nine people rescued on Tuesday night were people who got stuck driving through water that was deeper than it first appeared.

More than 200mm of rain has fallen in less than 24 hours along parts of the mid-north coast and weather experts say the relentless falls will likely continue until Thursday.

A severe weather warning is in place for dangerous flash flooding and possible gale force winds from Gosford to Port Macquarie.

Some streets in Dungog have been evacuated while sandbagging efforts are under way.

Parts of the Pacific Highway have been closed while motorists are urged to use extreme caution on parts that remain open.

The SES had been 1400 requests for assistance overnight with Newcastle, Dungog and Maitland among the areas hardest hit, spokeswoman Jacqueline Rose said.

Bureau of Meteorology forecaster Dmitriy Danchuk described the rain as “quite exceptional”.

The heaviest falls include 255mm recorded in less than 24 hours near Forster.

“On quite a few occasions around Newcastle we’ve picked up more than 200mm of rain since 9am Tuesday,” he told AAP.

“The rain is still continuing.”

The SES has again warned people to stay out of floodwater whether driving, riding or walking.

“It seems so simple but when people get there and see the water they just don’t seem to have a plan to go back,” Ms Rose told AAP.

The slow-moving low-pressure system, which has centred around Nelson Bay, is expected to move out to sea by Thursday morning, resulting in forecasts for “just a few showers” late in the week.