The Hunter Valley and Central Coast regions north of Sydney were declared disaster zones after being pounded by gale-force winds and torrential rains for a second day.

Massive seas ran aground at least 12 pleasure craft moored in Sydney Harbour, although fears eased that a massive coal freighter stranded at Newcastle would break up and create an environmental disaster.

Police put the official death toll at six, including a couple in their fifties whose car was washed off a bridge.

Searchers also found the bodies of four members of a family of five carried away when a highway collapsed.

The four bodies found were an adult female and three children -- a nine-year-old boy, a three-year-old girl and a two-year-old girl. The children's bodies were recovered.

Search teams were still looking for one more family member, believed to be an adult male.

The Australian Associated Press news agency reported that the adult body found, a 30-year-old woman, was the mother of the children and the missing male, a 30-year-old, was the father.

A Newcastle man was also missing after falling down a stormwater drain late Friday, police said.

With Australia beginning a long holiday weekend, officials advised motorists to scrap plans to travel to popular tourist spots in the state's north, saying a number of major roads were closed.

Power utility EnergyAustralia said 200,000 homes had no electricity and could remain blacked out until late next week.

"This is the worst storm and the worst damage our electricity network has seen for more than 30 years," EnergyAustralia network general manager Geoff Lilliss said.

Hundreds of people were evacuated as floodwaters rose, including 65 elderly residents of a Central Coast nursing home.

The Bureau of Meteorology said the storm dumped 300 millimetres (12 inches) of rain on some areas, with winds peaking at 120 kilometres per hour (75 miles per hour).

It said conditions had eased slightly Saturday morning, granting a reprieve to emergency services struggling to cope with more than 5,000 calls for assistance.

But meteorology bureau forecaster Julie Evans said that after causing extensive flooding in Newcastle, the storm was likely to intensify as it moved south towards Sydney before heading out to sea.

"It will be the sting in the tail in the system before it moves off to the east," she told Sky News.

Sydney ferry services were suspended for a second day amid huge swells.

Officials said at least a dozen recreational boats in Sydney Harbour had broken their moorings and run aground and that 30 more needed to be towed to safety, while a beachside wharf collapsed into to sea.

But the 30,000-tonne vessel Pasha Bulker, which ran aground Friday when the storm first hit, appeared to be structurally sound and there was no sign that its 800 tonnes of oil and fuel was leaking.