Four people were killed at the popular Dreamworld theme park in Australia's Gold Coast yesterday after a white-water rapids thrill ride malfunctioned.
Police said the victims were two women, aged 32 and 42, and two men, aged 35 and 38. Their names and nationalities have not been revealed.
The four-minute ride sends people on circular rafts down a foamy water track at speeds of up to 45kmh.
Witness Leah Capes told the Gold Coast Bulletin: "The kids and people in there were screaming. It all happened so quickly... There were heaps of people crying."
The authorities in Queensland said they were investigating the cause of the accident, which happened at about 2pm local time (noon Singapore time), but believe there was a problem with the conveyor belt on the Thunder River Rapids ride.
Two of the victims were thrown out of the raft, while the other two were trapped. "(The ride) had sustained some sort of malfunction, causing two people to be ejected... Another two people were caught inside the ride," Mr Gavin Fuller, a Queensland Ambulance Service supervisor, told reporters.
Some witnesses said there had been a problem with the ride earlier in the day and engineers had been called in for repairs. One witness told Channel Nine: "They drained all the water out and then had to refill it back up, and then we were allowed to go. We were stuck there for about 30 to 40 minutes at least."
Police would not comment on the cause of the malfunction.
Dreamworld is one of the country's best-known theme parks and attracts visitors from around the world. The park, which has operated since 1981, was closed following the accident.
It said in a statement: "Dreamworld is working as quickly as possible to establish the facts around the incident and is working closely with emergency authorities and police to do this."
Earlier this year, the park was closed after a person nearly drowned on another ride.
The accident involved a conveyor belt system similar to that used in the rapids ride, the Courier Mail in Queensland reported.
Australian Prime Minister Malcolm Turnbull last night offered condolences to the families of the victims. "This is a very, very sad, tragic event," he said in a statement.
The Queensland government provided counsellors to visitors who were at the park yesterday.
Said Queensland Premier Annastacia Palaszczuk: "This is the worst tragedy that we have seen at a theme park in Queensland in my history. We need to get to the bottom of this."