The Kimberley Ultramarathon is a RacingThePlanet 100 event, a series of 100 kilometre rough-country footraces. It is staged in some of the most remote and least explored places.
Each RacingThePlanet 100 event takes competitors through a variety of terrains bringing them into contact with unique geographies, local communities and historical landmarks.
The Kimberley region is one of Australia's top international and domestic tourist destinations. It features steep mountain ranges and sandstone and limestone gorges bordered on the west by the Indian Ocean.
Organizers said last month that it would attract 100 competitors from 30 countries. Its route covered outback areas between Kununurra, one of the three largest towns in the region, and the Gibb River Road to El Questro Wilderness Park.
The event meant to showcase Western Australia to the rest of the world has since been cancelled.
The Royal Flying Doctor Service flew the two grievously burnt women, one aged 35 and the other only 24, to Darwin hospital. Two men, aged 44 and 56, were flown to Perth.
Competitor Oskar Booth, 24, said the runners became trapped in a narrow gorge, with nowhere to hide.
"As we came out of checkpoint two, we came into a large amount of smoke but couldn't see any flames," Mr Booth said.
"The fire seemed to have accelerated and gone up to the gorge and trapped people.
"I could see thick plumes of smoke and it was getting hard to breathe - that's when I realised it was serious."
Race organiser, Hong Kong-based RacingThePlanet, posted a statement on its Facebook page saying: "Our thoughts are with the competitors who were burned yesterday in the bushfires.
"All participants involved in the event are off of the course and safe.
"We regret that we cannot provide more details at this time, but family members of those affected have been contacted."