Two men and two women have been killed in a ride malfunction at Dreamworld on the Gold Coast in what's being described as a "tragic incident".
A crime scene has been set up after the incident on the Thunder River Rapids Ride.
"We're now working together with the park to try and determine how this tragic incident has occurred," Todd Reid from Queensland police said at a press conference.
The four victims were believed to be aged from 32 to the early 40s. Reid would not confirm if the victims were related.
Queensland Ambulance's Gavin Fuller said two people were ejected from the ride and another two were caught inside the ride.
"The [injuries] were incompatible with life."
Counselling would be offered to ambulance officers who attended the scene, Mr Fuller said.
Dreamworld chief executive Craig Davidson said the park was working closely with emergency services to find out what went wrong.
"We are deeply shocked and saddened by this and our hearts and thoughts go to the families involved and to their loved ones," Mr Davidson said.
The Thunder River Rapids Ride is described on the Dreamworld website as a "moderate thrill."
Park-goer Lia Capes told ABC News she was about to go on the ride when she saw people running from it, 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 the area was quickly evacuated.
"The little girl that was crying, they asked us to take her because we distracted her, she was balling her eyes out," she said.
"[Then] people that worked here took her and her little sister away."