Australia's maritime safety watchdog says a tourist boat carrying 28 people appears to have crashed into a rock wall at Horizontal Falls.
The Falls Express — owned and operated by Horizontal Falls Seaplane Adventures — crashed just after 7am on Friday.
The Royal Flying Doctor Service spent most of Friday evacuating patients from the remote tourist attraction, 100km north of Derby.
Nine passengers remained in Royal Perth Hospital in a stable condition on Tuesday morning.
Australian Maritime Safety Authority national operations manager Greg Witherall said two inspectors had been at the remote site since late on Sunday.
AMSA says the vessel appeared to have crashed into a rock wall, contrary to initial reports it had capsized.
Initial investigations also confirmed that no passengers had been thrown into the water.
Mr Witherall said the inspectors would return to Perth to continue gathering witness accounts and information before handing the findings to a specialist investigation team.
"I would anticipate that in the next seven to 10 days we should have a pretty good idea of what actually occurred and the events that took place on that day," he said
Mr Witherall said the company could resume operations if the inspectors deemed it safe to do so once the special investigation team took over.
The safety authority had issued a direct notice to prevent the operator from using any other vessel to visit the tourist attraction pending the outcome of the investigation.
"These notices will be lifted once we've completed our preliminary investigation to ensure that there's no ongoing risks," he said.
"They certainly won't be operating unless all the risks, or anything that we've identified has been rectified."
Huge task for rescuers
Royal Flying Doctor Service emergency workers who were first on scene said the location's remoteness and lack of mobile reception added to the difficult task of rescuing patients.
The service set up a temporary triage zone at the nearby Koolan Island iron ore mine after deploying six aircraft, six doctors and six flight nurses to deal with the emergency.
Royal Flying Doctor Service doctor Sally Edmonds said she had not seen a "challenge" like the Horizontal Falls crash in her 20-year career.
"I've seen a lot of different injuries and illnesses with my time at the Royal Flying Doctor Service, but I've never been on a remote pontoon on the water with more than 10 injured people, that's really extraordinary," she said.
Dr Edmonds said there were people on the boat and pontoon when the she arrived.
"We could see that there were at least 15 people who [were] going to need medical attention, a lot of work had already been done with dressing wounds, stopping bleeding, and starting some pain relief," she said.
Flight nurse Natalie Saunders said there were "a lot of people everywhere".
"They had significant injuries, which we probably didn't expect," Ms Saunders said.
"We expected a lot of fractures, but not to the extent that people were hurt."
She said a lack of mobile reception with only one satellite phone was a barrier to the rescue.
"We couldn't communicate with our team on the ground on Koolan Island or Jandakot base, which made things quite difficult," Ms Saunders said.
"This was like one of those scenarios…a boat has crashed in a remote location, you have a nurse, a retired GP and a gastroenterologist and a pathologist there to assist you: what would you do? Go!"
Investigations continuing
While Horizontal Falls Seaplane Adventures has declined to provide any additional details of the accident, the company is offering refunds or rebookings to any customers impacted by their suspension in operations.
Mr Witherall said the authority would worth with state-level compliance agencies to piece together what happened leading up to Friday's crash.
"In the coming days we're going to be looking to have the vessel's mechanical systems tested, to determine if they have played a part in last Friday's accident," he said.
He said the authority would also be examining the prevailing weather conditions on the day and looking into the backgrounds of the staff involved in the incident as part of the investigation.
