Two men and two women have been killed on a ride at a theme park on Australia`s east coast.
Two victims were thrown from a raft on the Thunder River Rapids ride at Dreamworld on Queensland`s Gold Coast, while two others were trapped inside, officials said.
The park in Coomera has been closed and an investigation is under way.
Dreamworld bills itself as Australia`s biggest theme park with more than 50 rides and attractions.
The Thunder River Rapids ride whisks visitors in circular rafts along a fast-moving artificial river. Dreamworld describes it as a "moderate thrill" attraction.
Queensland Ambulance spokesman Gavin Fuller blamed a "malfunction" for the accident.
The victims have not been named but are reported to be two women aged 32 and 42 and two men aged 38 and 35.
Australia`s 9News reported that they were possibly from the same family.
Eyewitness Lisa Capes told Australian broadcaster ABC that she saw people running away from the ride 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.
Queensland police spokesman Todd Reid said investigators were reviewing CCTV footage of the incident while crews worked to remove the bodies from the scene.
"It is a complex retrieval involving heavy equipment and that will take several hours," he said.
He added he was not aware of any earlier problems with the ride, although that would form part of the investigation.
The theme park, 48km (30 miles) south of Brisbane, said it was "working as quickly as possible to establish the facts around the incident and is working closely with emergency authorities and police to do this."
Craig Davidson, head of Ardent Theme Parks, said they were "deeply shocked and saddened" by the incident.
Australian Prime Minister Malcolm Turnbull said he was "very saddened to learn of the tragic accident".
"Theme parks are a place for family fun and happiness, not tragedy," he said.
Worst amusement park accidents
Eight teenagers killed in a 1984 fire in US at the Six Flags Great Adventure amusement park in New Jersey. Arsonists believed to have started the blaze in "haunted house"
Six people died in failed simulated rocket launch in Shenzhen, China, in 2010. One of the cars on the centrifuge ride became loose, lost power and ride fell to the ground in flames with 44 people inside
Five children killed when Battersea Park Big Dipper malfunctioned in UK in 1972. Rope hauling cars to top of launch slope broke and safety mechanism failed, causing cars to roll into boarding area
Three died at Galaxyland Amusement Park in Alberta, Canada, in 1986. Last car on four-car train on the Mindbender triple loop ride came away from track, throwing off passengers before crashing into concrete pillar
One person died in rollercoaster accident at Expoland in Osaka, Japan, in 2007; another killed at Darien Lake theme park in New York State, US, in 2011
Sixteen people - including two teenage girls who needed leg amputations - injured at UK`s Alton Towers park in 2015. Engineers failed to notice a stationary car on the 14-loop Smiler ride and overrode the stop mechanism, sending the next car into it
Queensland Premier Annastacia Palaszczuk said a thorough investigation would be held into the "horrific, horrific scene".
"This has been an absolute tragedy, a tragic accident at Dreamworld where many thousands of people go on a daily basis," she said.