SYDNEY - HEAVY rains caused flooding across parts of eastern Australia on Saturday, forcing the evacuation of hundreds of people as rural towns throughout the area were put on flood alert.

A cyclone watch was also issued for northern Australia after a tropical cyclone that brought flooding rain and gale force winds to the city of Darwin moved eastwards, threatening isolated communities in the sparsely populated region.

Tropical cyclone Helen was downgraded to a storm after crossing the coast late on Friday, bringing down trees and flooding roads in Darwin, but the Bureau of Meteorology said it was likely to regenerate into a cyclone as it headed back out to sea.

More than 500 people were evacuated from houses and caravan parks, with some homes damaged by floodwaters, but early expectations of major flooding were downgraded, said State Emergency Services spokesman Catherine Moyle.

'We had been looking at a much greater number of evacuations but we're now doing localised evacuations as required,' she said.

Some people had been rescued after being trapped in cars, while one farming family had to be rescued after climbing up into the roof cavity of their home to escape the rising waters, she said.

There were no reports of deaths or injuries.

Some parts of northern New South Wales state received more than 300 millimetres of rain overnight, the Weather Bureau said.

Authorities said the rains had brought some good news, boosting water levels behind parched dams in a region that has suffered from persistent drought in recent years.

Australia regularly experiences cyclones between November and April, but forecasters say cooler than normal Pacific sea temperatures mean the current season could bring more cyclones than usual and increased rainfall.