Four people have been killed in an accident at a theme park on Australia's east coast, officials said.
Two men and two women died while on the Thunder River Rapids ride at Dreamworld, a popular attraction on the Gold Coast, Queensland state police officer Todd Reid told reporters.
A malfunction on the ride, which whisks people in circular rafts along a fast-moving, man-made river, caused two people to be ejected from their raft, while two others were caught inside the ride, said Gavin Fuller, an officer with the Queensland Ambulance Service.
He did not know if the two victims who were caught in the ride were trapped underwater, or caught up in the machinery.
Park staff administered first aid to the victims, but their injuries proved fatal, Mr Fuller said.
The victims were in their 30s and early 40s, he added.
Mr Reid said he was not aware of any previous problems with the ride.
Dreamworld chief executive Craig Davidson said the park was working with police to try to determine 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..
The theme park was closed following the accident.
Witness Lia Capes told the Australian Broadcasting Corp that she was just about to go on the ride when she saw people running out, crying.
"I was speaking to one of the guys and he said it was the raft or the boat thing in front of him - the whole thing flipped and everyone was screaming," she said.
Thunder River is considered one of Dreamworld's tamer, family-friendly rides, and is open to children as young as two.
The park, which has been open since 1981, also features several rollercoasters and the free-fall ride The Giant Drop, where passengers plunge from a height of nearly 400ft (120m).
In April, the park's Rocky Hollow Log Ride was temporarily shut down after a man fell from the ride.
Australian Prime Minister Malcolm Turnbull said there would be a thorough investigation into the cause of Tuesday's accident.
"Theme parks are a place for family fun and happiness, not tragedy," he said.
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