They suffered burns to up to 80 per cent of their bodies and were taken to Darwin Hospital before being flown to separate specialist burns units.
Turia Pitt, 24, arrived at Sydney's Concord Hospital on Sunday and is in a critical condition, while Kate Sanderson, 36, is also listed as critical at The Alfred Hospital in Melbourne.
Two men, aged 44 and 56, also suffered burns to up to 20 per cent of their bodies and are being treated at Royal Perth Hospital, where they are both listed as in a stable condition.
Royal Flying Doctor Service spokeswoman Joanne Hill told AAP there was a delay in getting to the victims because the area was so isolated.
"It took four hours to alert people to the fact that they needed help," Ms Hill said.
The ultra-marathon, which is part of the Racing the Planet Series, was being staged over 100 kilometres of outback between Kununurra, the Gibb River Road and El Questro Wilderness Park, from September 2 to 4.
Racing The Planet is a series of "rough country" endurance foot races set in some of the most remote locations in the world.
More than 40 competitors from 10 countries were taking part.
On the event's Facebook page, organisers said the race had been cancelled with all competitors removed from the course due to the bushfires in the area.
"Our thoughts are with the Kimberley Ultramarathon competitors who were burned yesterday in the bushfires," organisers said.Messages of support for the victims were also posted on Facebook.
Police and Fire and Emergency Services are investigating the cause of the bushfire.