Record flooding spreads in WA's Kimberley

"People in the Kimberley are experiencing a one-in-100-year flood event, the worst flooding WA has ever seen," Emergency Services Minister Stephen Dawson told reporters.

"This situation is still changing and it's proving to be extremely challenging."

About 60,000 cubic metres of water per second is flowing down the swollen Fitzroy River, which is expected to create a 50km-wide inland sea as it spreads across the flood plain.

Looma and Willare are also under threat after the river reached a record 15.81 metres upstream at Fitzroy Crossing, with dozens of people from other communities already evacuated.

"Multiple rescue missions have (also) been conducted in and around Fitzroy Crossing to rescue people from floodwaters," Mr Dawson said.

"As of today there have been 47 requests for assistance."

Road access to the town of Derby has been cut after flooding forced authorities to close a 700km section of the Great Northern Highway between Broome and Halls Creek, isolating the town of about 3000 people.

Authorities say it's likely the freight route south of Broome is also impassable at low-lying Roebuck Plain and it could take many weeks for it to drain.

Road trains from WA up to 53.5 metres will be permitted temporarily to travel through South Australia to ensure food and essential supplies reach WA's north and the Northern Territory.

"There are extensive resupply operations taking place today to ensure isolated communities are receiving essential supplies," Mr Dawson said.

Telstra's data communications cable has been damaged, blacking out parts of the Kimberley, after a major bridge suffered significant damage at Fitzroy Crossing earlier in the week.

Meanwhile, an Australian Defence Force C-130 Hercules plane has arrived in Broome, the first plane to land in two days, with more than 50 emergency workers on board.

A smaller C-27 Spartan plane is also expected to arrive in Kununurra to help with evacuations from Fitzroy Crossing and relief work, with another en-route to the Kimberley.

Weather conditions and the soggy airstrip at Fitzroy Crossing have hampered rescue efforts in recent days, but it has started drying out and the C-27, which can take off and land on a short runway, is set to speed up evacuation efforts.

The town remains surrounded by murky floodwater with two evacuation centres providing the community with support after the supermarket was inundated two days ago.

Dozens of homes have also been flooded and only a few streets remain above the waterline, with livestock and wildlife clambering for the remaining dry ground.

It comes as ex-tropical cyclone Ellie continues to dump heavy rain with strong winds on the West Kimberley, where up to 400mm was recorded in and around Broome over the past two days.

The slow-moving weather system is hovering near Broome and is expected to remain in the area for the remainder of Thursday before rapidly moving southeast towards the Northern Territory on Friday.