Ports, schools and roads have closed as Western Australia's Pilbara coast braced for Tropical Cyclone Zelia, bringing heavy rainfall and "hard to fathom" damaging winds.
"This is going to be a big one," WA Premier Roger Cook said.
The cyclone was looming 145km north of Port Hedland on Thursday afternoon after quickly developing into the maximum category-five system, with wind gusts up to 285km/h.
"That is the top of the scale. It doesn't get any worse than that," the Bureau of Meteorology's Angus Hines said.
"That is the most powerful tropical cyclone you can get, and it is forecast to be a category five system as it ... crosses the coast."
Cyclone Zelia is set to impact the coast on Friday night between Dampier and De Grey including Port Hedland, triggering flash flooding and wind gusts up to 290km/h.
"It's really hard to fathom wind that strong," Mr Hines said.
"That is strong enough to take out complete houses, trees, powerlines and cause widespread damage and disruption."
About a dozen schools, roads including the Great Northern Highway and major ports closed in WA's northwest as the region braced for impact.
"This is a dangerous system. It's big, it's strong and it's very unpredictable," Mr Cook said.
"People in the Pilbara need to be prepared, and they need to be prepared now."
Karratha local Carrie McDowell said the sky was "super black" in the town of about 22,000.
"We've not had a category five while lived here. We had a category three in 2020 and that was really scary," she told AAP.
"I'm a bit worried about flooding."
A cyclone warning has been issued for an area from Wallal Downs to Dampier, including Port Hedland, Karratha and Dampier, and extending inland to Marble Bar.
A "watch and act" alert is current from Eighty Mile Beach to Dampier, including Karratha and inland to Marble Bar and Millstream.
"There is a possible threat to lives and homes as a cyclone is approaching the area," the warning said.
Heavy rain is set to impact Wallal Downs to Wickham, spanning 450km, with some locations already recording up to 200mm with "a lot more" to come.
Flood watches have been issued across northwest WA, with the De Grey catchment and Pilbara coastal rivers stretching to the far northwest considered key areas of concern.
The cyclone is set to trigger more than 500mm of rain for some areas, with reports of a road train being washed off a bridge at Marble Bar late on Thursday.
Storm surges are also expected to greatly elevate tides at coastal locations near where the cyclone crosses, including around Port Hedland
Rio Tinto, BHP, Fortescue halted exports as Pilbara ports closed.
Rio Tinto said it was too early to predict how long port and rail operations would be shut down.
Meanwhile, the clean-up is continuing in north Queensland after record rainfall eased following almost two weeks of flooding that claimed two lives.
More than $100 million in state-federal grants are now available for flood hit primary producers, small businesses and not-for-profit agencies, with the Queensland government also donating $1.5 million to charity for victims.
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