Police say it is almost a miracle two children survived the fatal Thunder River Rapids tragedy at Dreamworld that left four people dead, as a union says it raised concerns about the operation and maintenance of some equipment at the park more than a year ago.
Queensland police have confirmed two flumes -- large, circular flotation devices -- collided at the conclusion of the Thunder River Rapids Ride, causing one to flip and tossing some of the occupants backwards onto the conveyer belt.
ACT woman Kate Goodchild, her brother Luke Dorsett, his partner Roozbeh Araghi, and Sydney mother Cindy Low were killed.
Two children, a 10-year-old boy and 12-year-old girl, were tossed from the ride.
"In terms of how they escaped, maybe through the providence of God or somebody, but it seems from what I've seen almost a miracle that anybody came out of that," Queensland Police Assistant Commissioner Brian Codd told media on Wednesday.
"If we're going to be thankful for anything, I'm thankful for that."
Police initially said two young girls escaped.
Earlier, Kim Dorsett, mother of Goodchild and Dorsett, told The Sydney Morning Herald she was "absolutely devastated".
"My whole family has been wiped out," she said.
Families have been leaving flowers outside the theme park, which remains closed on Wednesday.
Queensland Premier Annastacia Palaszczuk has said the children were being treated at Gold Coast University Hospital.
"They are being cared for at the moment and we are making sure that family members are contacted as we speak," she said.
A team of 30 detectives and specialist engineers are scouring the site of the 30-year-old ride, while police will probe Dreamworld's policies, procedures, maintenance schedules and the make-up of the ride to determine its compliance with contemporary engineering standards.
On Wednesday the Australian Worker's Union said it had raised concerns about ride operation issues at the park for more than a year before the accident.
"We did hold some very grave concerns about safety of equipment and the operation of equipment," AWU Queensland secretary Ben Swan told The ABC.
He said the union had lodged its first complaint about safety 18 months ago with park owner Ardent Leisure after a separate industrial incident.
Amusement park ride engineer David Eager, who sits on the Standards Australia Amusement Rides and Devices Committee, told the ABC on Wednesday that Dreamworld performs maintenance on its rides on a daily basis.
"As a general rule, you can keep them going forever by diligent and appropriate maintenance," he said in response to a question about ageing rides.
"And you might change the whole ride completely over that period. Like, an annual maintenance might be a total strip-down, you look at all the pins, the bolts, make sure there's no wear and tear, no crack, you do non-destructive testing on them."
He said a rides were generally only gotten rid of when something better came along
"But this is one of those rides that's a family ride. It's not a roller-coaster -- that's the biggest, it's the fastest. (This is) a river ride where people splash around on a hot summer's day in Queensland and just enjoy it," he said.
The Thunder River Rapids Ride reportedly broke down twice before the accident. In April this year, a man received cuts to his head and was hospitalised after being thrown from a 30-year old Log ride.
Past visitors have written on park's Facebook page to complain about their experiences at the park, while others have defended it.
"RIP and thoughts with the families, park staff, emergency workers and witnesses, devastated," wrote one user.
"This tragedy has hit us hard, myself and my 5 children have all been on this ride many times over the 30 years that it has been operating, the first time back when it first opened.
Another wrote: "seriously how many more years are you going to operate these extremely outdated rides for?? With so many complaints about safety and rides always closed for maintenance or because they are 'broken' you would think it's time for it to be demolished."