SYDNEY (Reuters) - A water ride that killed four people at an Australian amusement park in one of the world’s deadliest theme park accidents had completed an annual safety inspection less than a month ago, the park’s owner said on Wednesday.

Police were examining the Thunder River Rapids Ride at Dreamworld, near Gold Coast in Queensland state, where the four adults died Tuesday after being trapped under an upturned raft.

The owner of the park, Ardent Leisure Group, said the ride had completed its annual mechanical and structural safety engineering inspection on Sept 29. “As per regulations, this safety audit was conducted by a specialist external engineering firm,” Ardent said in a statement. The ride, meant to simulate going over river rapids, uses round floating devices that seat six and can reach speeds of 45 kph (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 two children, aged 10 and 13, free and trapping four adults beneath it, Queensland assistant police commissioner Brian Codd told reporters. He said the adults became caught in the ride’s conveyor-belt machinery.

Police have called for witnesses to come forward.

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 children to any of the adults.

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Their deaths follow a similar accident at Dreamworld in April, when a 19-year-old man was seriously hurt after falling from another water ride.

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.

“There is a very persistent and long process of representation by my union to Dreamworld and to the division of workplace health and safety around safety issues at Dreamworld,” AWU state secretary Ben Swan told Reuters.

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.”

Forensic police and workplace safety authorities were checking CCTV footage and will prepare a report for the state coroner.

Shares in Ardent 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. By the close of trade the stock had pared some losses to finish 14.9 percent lower.

The park is scheduled to reopen on Friday, with all the proceeds to be donated to the Australian Red Cross, Ardent said.