In this image made from video, rescue vehicles are parked outside Dreamworld on the Gold Coast, Australia, Tuesday, Oct. 25, 2016. Four people were killed while on a river rapids ride at the popular theme park on Australia's east coast. (Australian Broadcasting Corporation via AP)

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 , 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.

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Source: Reuters