Two of the victims were thrown clear of the ride while the other two were stuck in the ride itself.
Queensland Ambulance senior operating officer Gavin Fuller says all four suffered fatal injuries and couldn't be revived despite several crews attending the scene.
Mr Fuller says the ride suffered a malfunction which caused the accident.
"One of the rides have sustained some sort of malfunction causing two people to be ejected from the ride, another two people were caught inside the ride, two males and two females. They were assessed by QLD Ambulance personnel and had all sustained injuries that were incompatible with living."
It's unconfirmed whether the four victims are related with police refusing to reveal their identities until next of kin have been contacted.
Their ages range from 32 to early 40s, Mr Fuller told reporters.
Dreamworld chief executive Craig Davidson says the thoughts of the park were with the families and loved ones of those killed.
"At this stage the park is closed and we will work closely with police and emergency authorities in this incident. We're all deeply shocked and saddened by this, and our thoughts go to the families involved and to their loved ones."
QLD Inspector and regional duty officer Todd Reid says specialist police and forensic experts have arrived to investigate.
"I am able to say that investigations are continuing and we have had forensic pathologists attend the scene and examine the deceased. I am able to say the state coroner has attended the scene and examined the scene, that we have specialist police here engaged with witnesses and those people are being offered support as well as responding police."
Owners Ardent Leisure announced the park would be closed on Wednesday, while their shares fell 7.8 per cent on news of the tragedy.
Gold Coast Mayor Tom Tate has told the ABC the Gold Coast is known as a holiday destination, and the accident is a tragedy for the loved ones of the victims and for those working in the industry.
"This is one of the saddest days of our community to have this incident occur. The Gold Coast is renowned to get people here and to have wonderful experiences and memories to take back with them and this is the reverse of that. And I feel so sorry for the family. The staff of Dreamworld are in shock because they work as a family, the ambulance are working hard untangling everything, it's a very emotional day indeed."
Prime Minister Malcolm Turnbull says it's a very sad and tragic event.
"Our thoughts and prayers are with the families of those who lost their lives. It is a very sad day and we trust there will be a thorough investigation of the causes of this accident over the days that follow."
Thousands of shocked patrons flooded from the park after it was closed in the aftermath of the accident.
The Thunder River Rapids Ride is currently Australia's only river rapids ride and opened in December 1986.
It seats six people and travels up to 45 km per hour through rapid water.
Dreamworld's Rocky Hollow Log Ride was shut down in April this year when a man fell from the ride and nearly drowned.
