Two women and two men died when two rafts on the Thunder River Rapids ride at the Gold Coast tourist attraction.
Two women and two men died when two rafts on the Thunder River Rapids ride at the Gold Coast tourist attraction collided Tuesday, tipping one backwards, with police saying the victims were "caught in machinery".
But two girls on the six-person circular raft, aged 10 and 13, survived. Reports, which police would not confirm, said they were the daughters of one of the dead women.
"It seems, from what I've seen, almost a miracle that anybody came out of that," Queensland Police assistant commissioner Brian Codd told reporters after viewing CCTV footage of the tragedy.
"If we're going to be thankful for anything, I'm thankful for that."
He said it had been "absolutely traumatic" for the children who were now being cared for by family, as floral tributes were laid outside Australia's largest theme park, which is hugely popular with domestic and overseas tourists.
Queensland Ambulance officials said on Tuesday the ride malfunctioned and Codd said a thorough investigation was underway to find out what went wrong.
Ben Swan, the Queensland secretary of the Australian Workers Union, said the organisation voiced serious concerns about the operation and maintenance of some equipment at Dreamworld last year, although not the river rapids ride.
"I don't want to inflame the situation," he told the Australian Broadcasting Corporation.
"However, we do think that in the interests of workers at that facility, but also to the patrons of that facility... it is important there is a full inquiry."
The Australian newspaper reported that a series of mechanical problems were plaguing the water ride hours before the accident.
Codd said he was aware of the reports and added: "I would imagine that would be an important area of interest for the coroner".
The Gold Coast, which hosts the Commonwealth Games in 2018, is a major tourist destination and Queensland Premier Annastacia Palaszczuk pleaded with holidaymakers not to turn their backs on the area.
"The Gold Coast is safe," she told the Seven Network.
"Please don't alter your holiday plans. Please continue to show your support. We will get to the bottom of this."
Dreamworld, which opened in 1981 and has over 40 rides and attractions, said in a statement late Tuesday its whole team was "devastated and shocked".
"We are working closely with the authorities to understand exactly what occurred," it said.
The park remained closed Wednesday.