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

Two women and two men, who all lived in Australia, died when two rafts on the Thunder River Rapids ride at the Gold Coast tourist attraction collided on Tuesday, tipping one backwards. Police said the victims were "caught in machinery".

But a boy and a girl aged 10 and 12 on the six-person circular raft survived. Reports that 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 the incident had been "absolutely traumatic" for the children, who were being cared for by family, as flowers were laid outside Australia's largest theme park.

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

"We are just devastated, absolutely devastated," Mrs Kim Dorsett told Brisbane's Courier-Mail newspaper. "I have three children and have lost two of them - my whole family has been wiped out."

The New Zealand government confirmed that one victim was a New Zealand citizen.

Queensland Ambulance officials said on Tuesday that the ride, marketed as a family-friendly thrill, had malfunctioned.

Mr Codd stressed that a thorough investigation was under way. Thirty detectives were at the scene yesterday, with the 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.

Mr Ben Swan, the Queensland secretary of the Australian Workers Union, said the union had voiced serious concerns about the operation and maintenance of some equipment at Dreamworld last year, although not the river rapids ride.

"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," he said.

The Australian newspaper reported that a series of mechanical problems had plagued the water ride hours before the accident.

Mr Codd said he was aware of the reports, adding: "I would imagine that would be an important area of interest for the coroner."

Dreamworld remained closed yesterday and today. It will reopen tomorrow and plans to hold a memorial service for the victims.

AGENCE FRANCE-PRESSE, REUTERS, BLOOMBERG