Two children had a "miracle" escape when a raft flipped at an Australian theme park killing four adults, police said Wednesday, as claims emerged that safety concerns had been raised at Dreamworld last year.

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 a boy and a girl, aged 10 and 12, on the six-person circular raft survived. Police initially identified them as two girls.

Reports, which police would not confirm, said they were the children 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.

Two of those who died were identified in local media as Canberra-based brother and sister Kate Goodchild, 32, and Luke Dorsett, 35, with their mother, who was holidaying with them, distraught.

"We are just devastated, absolutely devastated,” Kim Dorsett told the Brisbane Courier-Mail.

"I have three children and have lost two of them -- my whole family has been wiped out."

Queensland Ambulance officials said on Tuesday the ride, marketed as a family-friendly thrill, malfunctioned and Codd stressed a thorough investigation was underway to find out what went wrong.

Thirty detectives were at the scene with rafts taken away by a forensic team.

"There will be a broad range of things examined -— policies, procedures, maintenance schedules and the actual, I guess, the make-up of the whole ride itself in terms of compliance about contemporary engineering," he said.


'Gold Coast is safe'

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."

The park remained closed Wednesday.