A brother and sister were among four adults who were killed when a river rapids ride malfunctioned at a popular theme park on Australia's east coast.
Police said two young children were fortunate to be alive after the accident at Dreamworld, a park on Queensland state's Gold Coast.
The Thunder River Rapids ride whisks people in circular rafts along a fast-moving, artificial river, with a conveyor belt helping move the rafts through the water.
Closed-circuit television footage showed the ride was coming to its conclusion when two rafts collided, Queensland Police Assistant Commissioner Brian Codd said.
"One has flipped backward and it has caught and tossed some of the people that were on the ride backward into the conveyor belt," he told reporters.
The two children, a 10-year-old boy and a 12-year-old girl, who shared the raft with the victims were thrown clear and managed to escape, he said.
"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."
Kim Dorsett, of Canberra, confirmed that two of the victims were her children - Kate Goodchild, 32, and Luke Dorsett, 35.
"I have three children and have lost two of them today -- my whole family has been wiped out," she told The Courier-Mail newspaper.
She was on a family holiday with her children and Goodchild's daughters from Canberra.
Police have declined to identify the other two victims, a 38-year-old man and a 42-year-old woman.
Media reports say the man was from Canberra and the woman was a New Zealand citizen who lived in Sydney.
Police will investigate reports of problems with the ride earlier on Tuesday, as well as maintenance records and procedures.
Dreamworld will remain closed as a crime scene for two or three days. Charges could follow including criminal negligence, Codd said.
Dreamworld chief executive Craig Davidson said the park was working with police to try to find out what went wrong.
"We are deeply shocked and saddened by this, and our hearts and our thoughts go out to the families involved and to their loved ones," he said.
Thunder River is considered one of Dreamworld's tamer, family-friendly rides, and is open to children as young as two. The park has been open since 1981.