Four people have been killed in a horror accident on a Gold Coast theme park ride. The two men and two women, aged between 32 and their early 40s, were killed while on the Thunder River Rapids ride at Dreamworld about 2.20pm Queensland time. Dreamworld chief executive Craig Davison said the park was closed and Dreamworld was "working to establish facts around the incident." He said he was "deeply shocked and saddened", and "our hearts and thoughts go out to the families involved". Queensland Ambulance Service spokesman Gavin Fuller said a malfunction caused two people to become ejected from the ride and another two were caught inside it. They were assessed by ambulance staff but had "sustained injuries that were incompatible with living". He said ambulance staff were "deeply affected" by the accident, and staff would be offered counselling. He couldn't be more specific on the nature of their injuries or how they died. Park staff performed first aid on the victims but they were unable to be saved. Queensland police spokesman Tod Reid said the forensic crash unit and workplace health and safety were at the scene of the accident. He said police were working with Dreamworld to determine what happened. Reid said he wasn't aware of any earlier issues with the ride, and couldn't say if the four were related. The Thunder River Rapids Ride is a water-based raft ride using a conveyor belt. The riders were understood to be trapped in the conveyor belt. A witness told the Courier Mail that her sister and niece were on the ride. "They are so traumatised, there is a woman hanging by her foot, crushed from the ride." Another said the ride flipped at the very end of its circuit. A man told the Courier Mail a girl was pulled from the ride just before it flipped. He said he thought there were four people on the ride at the time. Another witness Leah Capes said she was waiting near the ride entrance when people came running from the rapids ride line. "The kids and people in there were screaming. "It all happened so quickly... there were heaps of people crying." Gold Coast mayor Tom Tate issued a statement reflecting the community's shock at the news. "This is a very sad day for our city," Tate said. "Our thoughts are with the families of those affected - and the emergency staff in attendance." A House of Travel spokesman said the Gold Coast was "probably New Zealand's most popular short-haul family holiday destination". Nearly 190,000 New Zealanders travelled to the Gold Coast in the year up to March 2016. The same conveyor belt system used to operate the deadly Thunder River Rapids ride at Dreamworld was to blame for a near death on a similar ride at the park in April this year. The previous "serious incident" on the Log Ride nearly caused a man in his 30s to drown after the conveyor belt slipped and trapped the man. Workplace Queensland and independent inspectors were forced to give the ride the all-clear before it was able to reopen again.