The boat carrying two French tourists who died on the Great Barrier Reef has remained docked for the day after staff were offered psychological counselling in the wake of the rare double fatality.
The deaths of Jacques Goron, 76, and Danielle Franck, 74, at a popular snorkelling spot north-east of Cairns are being scrutinised by authorities.
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Snorkelling deaths 'almost unheard of'
Two French tourists have apparent heart failure at the same time while snorkelling on the Great Barrier Reef off Cairns. November 16, 2016
The tour company was said to have offered counselling to the pair's partners, who were part of a group of 21 French people on board.
Police were tight-lipped on the circumstances surrounding the deaths, believed to be a result of dual heart attacks.
"We'll treat any death as an investigation on behalf of the coroner, so until we actually know, and that may not be until after there's been a post mortem examination, we may not know what the cause of death was," Acting Superintendent Rolf Straatemeier said.
Workplace Health and Safety will investigate the deaths, which happened in front of their relatives and other holidaymakers not long after they got into the water.
Inspector Peter Mansfield told reporters in Cairns on Wednesday police would prepare a report for the coroner, including what treatment was given to the pair.
"Some of our inquiries will cover that, particularly in relation to their first aid," Inspector Mansfield said.
![Passions of Paradise has taken hundreds of thousands of tourists to the reef, with a fatality in 1997.](/web/20170507144934im_/http://www.brisbanetimes.com.au/content/dam/images/g/s/q/u/a/x/image.related.articleLeadwide.620x349.gsrfz7.png/1479365250281.jpg)
Sydney cardiologist Ross Walker told News Corp dual heart attacks due only to pre-existing medical conditions were highly unlikely, saying the holidaymakers were likely stung by irukandji jellyfish.
Inspector Straatemeier said police were unable to comment on that until after the post-mortem but Australia's marine tourism peak body rejected the theory.
"From my experience with irukandji, and I've had a couple of people from my crew who have been stung over time and they basically hardly know they've been stung," Association of Marine Park Tourism Operators executive director Col McKenzie said.
"It takes hours for it to develop and this is something that's happened within the first five minutes of these people entering the water. It just doesn't match."
Mr McKenzie said there were no reports of irukandji in the area.
Police said the female snorkeller, at least, was in the water for less than 10 minutes and Inspector Straatemeier said he wasn't aware of any reports of the deadly stinger either.
According to the Queensland Ambulance Service, irukandji syndrome symptoms are not immediate and may appear five to 45 minutes after the sting.
Mr McKenzie said he believed one of the tourists had been wearing a stinger suit, although that didn't rule out the possibility of them being stung on the hand or face.
He said the dive company, Passions of Paradise, had taken more than 400,000 tourists to the reef since if began operating in 1989.
"Passions of Paradise didn't run today and all of the crew were going for counselling with a clinical psychologist I don't know what's happened with family," he said.
Passions of Paradise chief executive Scotty Garden was unavailable on Thursday afternoon.
AAP