Two children are fortunate to be alive, police said Wednesday, after they were thrown clear and survived an accident that killed their mothers and two other people on a river rapids ride at a popular theme park in Australia.

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Two men and two women died in the accident on Tuesday at the Dreamworld park on Queensland state's Gold Coast, Queensland Police Assistant Commissioner Brian Codd said.

The 30-year-old Thunder River Rapids ride whisks people in circular rafts along a fast-moving, man-made river, with a conveyor belt helping move the rafts through the water. Closed-circuit television footage showed the ride was ending when two rafts collided, Codd said.

A 10-year-old boy and a 12-year-old girl were thrown clear as the six-person raft upturned and trapped the adults against the conveyor belt, he said.

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"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," Codd said. "If we're going to be thankful for anything, I'm thankful for that."

The children had been traumatized and were being cared for by family, he said.

Kim Dorsett, of Canberra, confirmed that two of the victims were her children: Kate Goodchild, 32, and Luke Dorsett, 35.

"I have two granddaughters - an 8-month-old and a 12-year-old - and it truly breaks my heart to know that my 8-month-old is never going to get to know her mum," she told The Courier-Mail newspaper.

She said the 12-year-old "is completely devastated -- she is blaming herself for what has happened."

Kim Dorsett was on a family vacation with her children and Goodchild's daughters from Canberra. Luke Dorsett's partner, Roozi Araghi, 38, of Canberra, was also killed.

The boy's mother was Cindy Low, a 42-year-old New Zealand citizen who lived in Sydney.

Codd said police would investigate reports of problems with the ride earlier on Tuesday, as well as maintenance records and procedures.

Dreamworld said the ride had passed its annual safety inspection a month ago.

The park statement said it would reopen on Friday, but "activities will be limited to smaller rides, animal attractions, and the water park."

Thunder River is considered one of Dreamworld's tamer, family-friendly rides, and is open to children as young as 2.