Two men and two women have been killed on the Thunder River Rapids ride at Dreamworld on the Gold Coast.
Two of the four victims were thrown off the ride when it malfunctioned and the other two were trapped inside.
Police Inspector Tod Reid said a 42-year-old woman, a 38-year-old man, a 35-year-old man and a 32-year-old woman had lost their lives.
Early investigations suggest water pushed one raft into another and one of the rafts tipped over, throwing the riders out.
"[The ride] had sustained some sort of malfunction causing two people to be ejected ... another two people were caught inside the ride," Queensland Ambulance Service senior operations supervisor Gavin Fuller said.
"Two males and two females, they were assessed by Queensland ambulance personnel and had all sustained injuries that were incompatible with living."
According to the Queensland Ambulance Service (QAS), injuries that are incompatible with living are those "where it would be appropriate to withhold the commencement of CPR".
Mr Fuller said the QAS had multiple units at the scene, including critical care paramedics, who arrived at Dreamworld to find park staff trying to render first aid.
"A number of the staff that were here today from Queensland ambulance have been deeply affected by this terrible accident that's occurred today," he said.
Dreamworld chief executive Craig Davidson said the accident happened at 2:20pm (AEST).
"At this stage, the park is closed and Dreamworld is working closely with police and emergency services and authorities to establish the facts around the incident," he said.
Mr Davidson said the company was "deeply shocked and saddened by this and our hearts and our thoughts go to the families involved and to their loved ones".
Video: The popular theme park was closed after the accident (ABC News)
Dreamworld has advised that the park will be closed until further notice.
The Thunder River Rapids ride is described on Dreamworld's website as a "moderate thrill".
"Travel down a foamy water track past the Gold Rush Country, speeding up to 45 km/h through the turbulent rapids," the website reads.CCTV footage available: police
Shortly after 7:00pm, Inspector Reid updated the media on the investigation, saying police were likely to remain at the scene late into the night.
"It is a complex retrieval involving heavy equipment and that will take several hours," he said.
He said forensic pathologists had attended the accident site.
"I'm able to say the state coroner has attended the scene and examined the scene," he said.
"There is CCTV footage available and that will be reviewed as part of the investigation."
Inspector Reid would not confirm whether any of the adults who died were related, saying police were in the process of notifying their families.
"We also have the people who witnessed that, having versions obtained from them at police stations nearby by trained investigators," he said.'Everyone was screaming'
Lia Capes said she was just about to go on the ride when she saw people running out, crying.
"We saw [a] little girl and we believe it was her mum because it was just her and her little sister that was an infant," Ms Capes said.
"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."
Ms Capes said Dreamworld staff quickly evacuated the area.
"The little girl that was crying, they asked us to take her because we distracted her, she was bawling her eyes out," she said.
"[Then] people that worked here took her and her little sister away."'Theme parks are for fun, not tragedy': Turnbull
Prime Minister Malcolm Turnbull also expressed his condolences and said he expected there to be a thorough investigation.
"Theme parks are a place for family fun and happiness, not tragedy," he said.
"This is a very, very sad and tragic event. Our thoughts and prayers are with the families of those who lost their lives."
Federal Opposition Leader Bill Shorten described the accident as "heartbreaking".
"Sending our love to the families of those who lost their lives in this devastating tragedy," he said.Premier took family on ride
Queensland Premier Annastacia Palaszczuk said she took her family on the rapids ride earlier this year.
She said while the theme park had a good safety record, there would be a thorough investigation.
"We want to leave no stone unturned to get to the bottom of this. We want to ensure that nothing like this ever happens again," she said.
"Safety needs to be a number one priority and what we've seen today is perhaps the worst tragedy in a theme park in my living memory."
Gold Coast Mayor Tom Tate told the ABC the accident had shocked everyone in the tourist city.
"It's a sad day for our city. The Gold Coast is one of those places renowned for people to come and take away the best memories of their lives, and this tragedy is the reverse of that," he said.
"It's a freakish accident and one of the ones that will be deep in all our memories.
"The message I want all Gold Coasters to know is that we will do as a community what we can to help the families.
"It's caught everyone by surprise and we're all in shock."
In April, emergency services said a man nearly drowned after he was thrown from the log ride at Dreamworld.
In September, several people had to be evacuated from a rollercoaster at nearby Seaworld during the school holiday season.
Nobody was injured when the rollercoaster stalled mid-ride.