Forensic police were examining a river rapids ride at Australia's biggest theme park on Wednesday after four people were killed when it apparently malfunctioned and they were trapped beneath an upturned raft.

Tuesday's tragedy at the Thunder River Rapids Ride at Dreamworld on the Gold Coast in the state of Queensland ranks among the world's deadliest theme park accidents.

"Our thoughts and prayers are with the families of those whose lives were lost in this terrible accident," Australian Prime Minister Malcolm Turnbull said on Australian Broadcasting Corporation radio on Wednesday.

Shares in park owner and operator, Ardent Leisure Group <AAD.AX>, plunged 21 percent at the open on Wednesday, adding to a 7 percent fall in the final hour of trading on Tuesday after the accident.

The ride, meant to simulate going over river rapids, uses round floating devices that seat six, and can reach speeds of 45 kmh (28 mph). It is described by Dreamworld as a "moderate thrill" attraction for those older than two.

A collision between two rafts flipped one, throwing free two girls, aged 10 and 13, and trapping four adults beneath it, Queensland assistant police commissioner Brian Codd told reporters on the Gold Coast. He said the adults became caught in the ride's machinery.

The victims, two men aged 33 and 38 and two women aged 32 and 42, all lived in Australia, police said. The New Zealand government confirmed one was a New Zealand citizen. Two children on the ride were being treated for injuries in hospital.

Police have not yet confirmed the relationship of the hospitalized children to any of the adults. Local media reported that a brother and sister were among the fatalities.

Forensic police and workplace safety authorities, who are also checking CCTV footage, will prepare a report for the state coroner, who will then decide if any charges should be laid. The park will remain closed indefinitely, its operators said.

The Australian Workers Union (AWU) said it had raised "grave concerns" about safety at the theme park with authorities and the park operator as early as April 2015.

"We did hold some very grave concerns about the safety of equipment and the operation of equipment at that site," AWU state secretary Ben Swan told ABC radio.

Asked on Tuesday whether there were any earlier problems with the ride, Tod Reid, an inspector with Queensland Police, said, "I’m not aware, but that will be part of the investigation."