WA residents in the 700 kilometres south of Broome down to Whim Creek are bracing for Severe Tropical Cyclone Ilsa, which is expected to cross the coast between Port Hedland and Wallal Downs tonight or early on Friday as a category 5 system.

Areas including Port Hedland and extending inland are expected to be affected.

Ilsa reached Category 4 earlier today, with sustained winds near the centre of 185kmh and gusts to 260kmh. It is located around 245km north of Port Hedland while heading south southwest at 16kmh.

"Ilsa is expected to turn to the southeast this afternoon and develop further, and is now forecast to reach category 5 intensity as it moves towards the east Pilbara coast,” the Bureau of Meteorology said.

Broome Insurance Brokers Director David Keys tells insuranceNEWS.com.au the large mine export facility at Port Hedland has been cleared, with its giant ships sent out to sea.

“The port was completely shut down, which they just never do, and they sent all the ships out to somewhere safe - the massive oil tankers and big cargo ships. The containers could probably easily be picked up and thrown around. It's almost unprecedented for it to be shut down like that so they're obviously worried by it,” Mr Keys said.

A severe cyclone impact will occur along the coast and adjacent inland parts to the east of Port Hedland and west of Wallal Downs, the Bureau says, maintaining tropical cyclone intensity on Friday as it tracks past Telfer and further inland across the northern interior district.

Extreme gusts up to 285 kmh at the core are expected to impact the area between De Grey and west of Bidyadanga first, before tracking inland, and very destructive wind gusts up to 170 kmh are possible at Telfer during Friday if the centre of Isla passes close by.

Gusts to 155 kmh may develop near the coast between Bidyadanga and Port Hedland before extending inland as far as Marble Bar tonight and to Telfer during Friday.

Squally thunderstorms are expected over the western Kimberley, with 300mm of rainfall possible near where Ilsa crosses the coast. Abnormally high tides are possible about the coast.

The Bureau had an active yellow alert for people between Bidyadanga and Whim Creek, including Port Hedland and extending inland to include Marble Bar, Nullagine and Telfer.

"People need to act and get ready to shelter from a cyclone,” it advised those residents.

A blue alert was active for people south of Broome to Bidyadanga (not including Bidyadanga), inland areas to Parnngurr and Kiwirrkurra.

Those residents “need to prepare for cyclonic weather and organise an emergency kit including first aid kit, torch, portable radio, spare batteries, food and water,” the Bureau said, adding that people in or near Beagle Bay to Broome, including Broome, have the “all clear”.

"It's looking more and more positive for Broome. It's now almost guaranteed to miss us,” Mr Keys said.

The system is expected to weaken below tropical cyclone strength overnight Friday as it moves east into southern parts of the NT.

The Australian Reinsurance Pool Corporation (ARPC) has declared Ilsa a cyclone event under the pool, which is run by ARPC and backed by a $10 billion government guarantee.