Rail safety investigators have arrived at the site of a deliberate train derailment in Western Australia's Pilbara region, a day after a runaway train brought iron ore miner BHP's rail network to a halt.

The fully-laden train, pulling 268 wagons of iron ore, was travelling from Newman to Port Hedland when the driver got out to inspect a wagon near Hester siding at 4:40am on Monday.

While the driver was out of the cabin, the train took off, travelling more than 90 kilometres in 50 minutes.

The train, made up of four locomotives, was eventually derailed at a set of points at Turner's Siding, about 120km south of Port Hedland.

Mobile phone footage obtained by the ABC shows the twisted and mangled wreckage of the train lying strewn alongside the track.

The derailment was orchestrated from BHP's Integrated Remote Operations Centre in Perth, about 1,500km away.

Two rail safety officers from the Office of the National Rail Safety Regulator have arrived at the site of the derailment to begin an investigation.

The Australian Transport Safety Bureau (ATSB) is separately investigating the derailment.

A spokesperson for the ATSB said the investigation was likely to be completed in the second quarter of 2019.

The train reached an average speed of about 110kmh before it was derailed. ( Supplied )

Out-of-control train 'very concerning': Premier

WA Premier Mark McGowan said while he had not been briefed on the incident, it was clear "extraordinary measures" had been required.

Sorry, this video has expired Aerial footage shows crash site after runaway train derailed

"Obviously it would have been very concerning for everyone involved — BHP and the workforce, and the people of Port Hedland — that a train was travelling down the track without a driver and obviously out of control," Mr McGowan said.

"I'm sure that there'll be a full review undertaken by BHP, and I'll consult about what role the state might have in that."

In a statement, BHP said about 1.5 kilometres of track was damaged, and the recovery process would take about one week.

"We are working with the appropriate authorities to investigate the situation," the company said.

In the meantime, train operations across BHP's network have been suspended.

The derailment was orchestrated from BHP's remote operations centre in Perth. ( BHP Billiton )

Data recorder may hold answers

Based on the distance travelled, the runaway train reached an average speed of about 110 kilometres per hour.

Rail Safety Consulting Australia owner Phillip Barker said it was unclear what had happened to make the train take off, but suggested it may have involved the configuration or isolation of the brake system.

BHP is counting the cost of the derailment, with analysts saying it will run into the millions. ( ABC News )

"Usually, once the driver leaves the train, the brakes are on, there's procedures for that," Mr Barker said.

"They would have got permission to get off the locomotive and go and look at this wagon.

"There would have been a procedure in place to make sure the train didn't move off."

The clean-up operation at the crash site is expected to take about a week. ( ABC News )

Mr Barker said rail safety officers would begin by asking the driver and rail controllers what happened.

"They'll put a notice on [BHP] to isolate the data recorder … to see what the parameters were, what was happening, especially with the brakes," he said.

"Any traction power that may have been applied, what the brake pressures along the train — on the front and the back — may have been.

"It's an iterative approach and they'll start working from the bottom, at the accident [site], right up to the top.

It remains unclear exactly how the train managed to take off without its driver. ( ABC News )

"They'll be looking also at the safety systems … to see if they're in place, if they're adequate and if they were followed."

Don't jump to conclusions: union

The union representing mining workers said it was important to wait until investigations were completed before blaming anyone over the incident.

Iron ore reserves at Port Hedland will be used by BHP while rail operations are suspended. ( Supplied by BHP Billiton )

CFMEU WA secretary Greg Busson said they would await the findings of the inquiry.

"Everyone should just let the investigation take its course and not jump to conclusions, poking blame here or there," he said.

"I think if the investigation's done properly, that'll find the true blames, and as long as we use the outcomes of it to make the industry safer then all the better."

The union said it was lucky there were no other trains in the area and that the incident did not occur closer to Port Hedland, where there could have been tragic consequences.

BHP will use its stockpile reserves to maintain supply at its port operations in Port Hedland while rail operations are suspended.