Military helicopters airlifted hundreds of people from communities cut off by "once in a century" floods in Australia's northwest, an official leading relief efforts said on Sunday, noting water covered some places "as far as the eye could see".

The crisis in the Kimberley - an sparsely populated area in Western Australia state about the size of California - was sparked last week by severe weather system Ellie, a former tropical cyclone that brought heavy rain.

"The water is everywhere," Western Australia Emergency Services Minister Stephen Dawson told reporters in Perth.

"People in the Kimberley are experiencing a one-in-100-year flood event, the worst flooding Western Australia has had in its history."

In some parts, he said flood waters stretched for 50 kilometres with inundation "as far as the eye can see".

The emergency comes after frequent flooding in Australia's east over the last two years due to a multi-year La Nina weather event.

Some eastern regions have endured four major flood crises since last year caused by the La Nina system, which is typically associated with increased rainfall.

The town of Fitzroy Crossing, a community of around 1,300 people, has been among the worst hit, with supplies having to be airlifted in due to flooded roads.

Across the Kimberley, where around 50% of residents are Aboriginal, 233 people had so far been evacuated due to flooding, authorities said.

The Bureau of Meteorology said on Sunday that rain had eased as the storm shifted eastwards to the Northern Territory, but warned that "record-breaking major flooding" continued in the Kimberley.

"Many roads are impassable and many communities are now isolated," the forecaster said on its website.

The Fitzroy River hit 15.81 metres (52 feet) at Fitzroy Crossing on Wednesday, breaking its 2002 record of 13.95 metres, a bureau spokesperson said.

State emergency authorities have warned residents in other small communities of rising water in the region, which includes the resort town of Broome, about 1,240 miles (2,000 km) north of Perth.

While the extent of flood damage was difficult to assess, authorities expected the recovery effort to take months.

Prime Minister Anthony Albanese on Saturday described the flooding as "devastating" and pledged federal assistance.

Australian Defence Force (ADF) aircraft were being used to assist flood-hit communities, and Chinook helicopters were en-route to help relocate residents, according to authorities on Saturday.

Five ADF helicopters will start operations in the Kimberley by Thursday, a defence spokesperson said.