UPDATED 3.01PM: Wild weather is set to lash the New South Wales coast in Australia for the rest of the day, with insurers declaring a catastrophe for affected areas and interstate crews joining relief efforts.

The names of the three people who died in the floods in the NSW town of Dungog have been released by authorities.

79-year-old Colin Webb, 68-year-old Robin MacDonald and 72-year-old Brian Wilson were all believed to have been trapped in their residences after a flash flood swept through the town in the Hunter region yesterday.

The men were both believed to be living at a retirement village.

Two people are feared drowned as rescue teams search for a car swept into a creek in the same area.

Witnesses reported two cars were swept into Fishery Creek at Maitland about 9am today.

Police say a silver hatchback was swept away and is submerged, and a man jumped into the flooded creek to try and help two people from one of the cars.

Those three people have been rescued and are being treated for hypothermia, but people from a second car haven't been located.

Insurance companies have received almost 20,000 claims, with losses estimated at AU$129 million so far.

About 200,000 homes and businesses across Sydney, the Hunter and Illawarra are still without power.

The weather bureau says a second storm cell off Newcastle is expected to bring more thunderstorms today.