Torrential rain soaked the Coffs Harbour region north of Sydney overnight, swamping the arterial Pacific Highway with flash floodwaters that isolated almost 5,000 people, emergency officials said.

About 40 people had to be evacuated from the area hit by the raging floods and New South Wales emergency services minister Steve Whan declared a natural disaster, releasing state funds.

More than 500 millimetres (20 inches) of rain had fallen in the past two days, Whan said, in the fifth major flooding incident to hit the region this year.

"I guess one of the things we've seen predicted from climate change consistently is that the rain and the weather events will come in more storms and more short-term deluges," he said.

"Unfortunately that's the pattern that we seem to be seeing this year in the area."

Floodwaters were expected to peak at five metres (yards) at Coffs Harbour on Saturday afternoon before easing, the State Emergency Service said, describing as "drastic" the cumulative effect of recent downpours.

"Weather conditions have eased considerably over the last few hours. However, we still have some 4,800 people isolated by floodwaters in a number of north-coast communities," said SES spokesman Phil Campbell.

Intense storms hit the east coast late last month, generating more than 10,000 lightning strikes and disrupting train and flight services.

Tens of thousands of homes and businesses were blacked out and a man was killed when his car hit a tree in torrential rains.

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