Trapped ... Turia Pitt (L).

Trapped ... NSW runner Turia Pitt (L) was severely burnt by the fire.

An ultramarathon runner, who was trapped in a bushfire in Western Australia's Kimberley region, has described how he and three other competitors leapt through a wall of flames to save their lives.

The four runners had been competing in the 100-kilometre Kimberley Ultramarathon, part of the Racing the Planet series, on Friday afternoon when the bushfire crossed their path.

[We] had a quick choice of being a human fireball and burning and that was the end, or what we did... 

They became trapped in a gorge by a bushfire that had been burning in the El Questro Wilderness Park.

Kate Sanderson

Kate Sanderson

Victorian runner Kate Sanderson, 35, and New South Wales woman Turia Pitt, 24, were in critical conditions in hospitals in Sydney and Melbourne, with burns to 80 per cent of their bodies.

Two men, aged 44 and 56, also suffered burns to about 20 per cent of their bodies and were being treated locally in Royal Perth Hospital.

One of the men, Michael Hull, of Gosford in NSW, described to the ABC the moment he and the other runners had to confront the wall of flame.

"We got to a situation where the flames were about two metres away from us," Mr Hull said.

"It was just horrendously hot and we were scared for our lives."

He and the others tried to escape the blaze but he said it enclosed around them creating a wall of fire.

"[We] had a quick choice of being a human fireball and burning and that was the end, or what we did, instantaneously, is just stop and run back through the wall of flame," he said.

"We knew the flame, while it was high, we knew on the other side of it there was nothing left to burn."

They leapt through the flames and other competitors came to their aid, he said.

Ms Sanderson, from Mornington, was flown from Darwin to Melbourne on Saturday night and is being treated in The Alfred hospital for burns to more than 70 per cent per cent of her body.

Ms Pitt, who was originally from Ulladulla on the New South Wales south coast and worked as an Argyle diamond mine engineer, was also in a critical condition at Sydney's Concord Hospital.

A friend of Ms Pitt's said it took three hours for the burnt competitors to be rescued but despite the set back Ms Pitt had shown hopeful signs.

"Turia has had a second operation and has recently been taken off breathing ap and is breathing on her own," the friend said.

"Friends and family are full of hope as she is a very strong and determined girl."

A spokeswoman for Kununurra Hospital, where the runners were first treated, said race organisers flew the victims to the hospital in a helicopter before the Royal Flying Doctor Service took them to Darwin Hospital early on Saturday.

Racing The Planet event organiser Samantha Fanshawe told the ABC an investigation was underway into how the runners came to be trapped by the fire.

Ms Fanshawe would not give details of the investigation but said organisers were told there was no chance of a fire occurring on the course.

"We are looking into every detail of the incident, it was a terrible day and a terrible thing to have happened," she said.

"It was very scary for the people trapped in that gorge. ...Everyone we spoke to prior to the race said there was no risk on the course to fires.

"This fire flared up very suddenly, part of the investigation is putting together a full timeline and also looking at everybody who was involved in the pre-race part. So at the moment I don't want to disclose those details."

Ms Sanderson was a well-known member of the Victorian ultra-running and orienteering communities and recently competed in the North Face 100 race.

According to her competition biography, Ms Sanderson had completed five 100 kilometre ultramarathons, although the Kimberley race was her first with the Racing the Planet group of races.

Australian Ultrarunners Association secretary Brett Saxon said ultramarathon runners knew the risks of their sport - but would not expect bushfire.

''We run in alpine regions so there's a lot of risk from snakebites, that sort of thing,'' he said.

''But fires at events, I don't think you fear will happen to you.''

- With Rachel Olding, Dewi Cooke and AAP