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, who were aged from 32 to their early 40s, were thrown off the ride when it malfunctioned and the other two were trapped inside.
"[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.
Mr Fuller said the service had multiple units at the scene, including critical care paramedics, who arrived at the site in Coomera to find theme park staff trying to give first aid.
A number of ambulance staff had been deeply affected by the "terrible accident", he said.
Dreamworld chief executive Craig Davidson said the accident happened about 2.20pm (5.20pm NZT).
"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".
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.
It said up to six people could be seated per ride and riders must be 120cm tall.
Queensland Police Inspector Tod Reid said a crime scene had been established.
"Workplace health and safety officers and the forensic crash unit are in attendance, along with the state coroner," he said.
"We're now working together with the park to determine how this tragic incident has occurred."
He would not confirm whether any of the adults who died were related, saying their identities had not been confirmed.
He was also unable to say whether the ride had shown any signs of problems earlier in the day, but said that would be part of the investigation.
'Everyone was screaming'
Lia Capes said she was just about to go on the ride when she saw people running out, crying.
"We saw 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."
In a post on Facebook, Gold Coast Mayor Tom Tate said it was "a very sad day for our city".
"Our thoughts are with the families of those affected - and the emergency staff in attendance," he wrote.
"I urge everyone to show the respect needed as the relevant authorities undertake investigations into what has occurred."
- ABC / RNZ