The death of four people at a Gold Coast theme park has been labelled one of the saddest days in the city's history.

Two men and two women died at Dreamworld when the family favourite Thunder River Rapids ride apparently malfunctioned on Tuesday afternoon.

The exact circumstances of the accident are under investigation but the six-person raft the victims were on flipped over while travelling on a conveyor belt.

Two of the victims were flung clear of the raft into the water, while the other two were trapped under the raft.

Supplied

The tragedy left the city stunned, rocking its image as a destination for fun-filled family holidays.

"It's one of the saddest days in our city," Gold Coast mayor Tom Tate told AAP.

"The Gold Coast is renowned for people to come here and have wonderful memories for the rest of their lives.

"This is the reverse of that. A tragedy like this, it affects us deeply because we cherish making sure we look after our visitors.

"It is the most emotional day we've had."

Queensland Premier Annastacia Palaszczuk said: "I understand there were many witnesses and I want to thank paramedics and police who were there.

"I have been told the scene was horrific."

I'm very saddened by the tragedy at Dreamworld today. Our thoughts and prayers are with the families. https://t.co/FGF9vsrB1s — Malcolm Turnbull (@TurnbullMalcolm) October 25, 2016

Police wouldn't release the identities of the victims, but said they were a 32-year-old and 42-year-old women, and 38-year-old and 35-year-old men.

It is believed that at least two of those killed are related, and other media reports have claimed some or all of the victims were on holiday from Canberra.

Police were in the process of notifying the victim's families early on Tuesday night, and were talking to witnesses at nearby police stations as the bodies began to be recovered at the park.

Heartbreaking news from Dreamworld. Sending our love to the families of those who lost their lives in this devastating tragedy. — Bill Shorten (@billshortenmp) October 25, 2016

"It is a complex retrieval involving heavy equipment and that will take several hours," Inspector Tod Reid said.

Footage of the incident has also been captured on CCTV cameras.

"That will be reviewed as part of the investigation," Inspector Reid said.

One witness described consoling a young girl at the scene in the aftermath of the accident, who she fears was related to at least one of those killed.

The thoughts and prayers of all Qlders are with the family and friends of those who lost their lives today at Dreamworld. — AnnastaciaPalaszczuk (@AnnastaciaMP) October 25, 2016

"We saw a little girl screaming for her mum ... it was horrible," Lia Capes told Seven News.

"It was so scary.

"We tried to comfort her to the side to distract her."

Several parkgoers claimed the ride had broken down at least twice earlier in the day and police said those claims would form part of the investigation.

"I'm not aware (of earlier problems on the ride) but that will obviously be part of the investigation," Inspector Reid said.

Queensland Ambulance senior operating officer Gavin Fuller said park staff had attempted first aid on the victims but their injuries were just too severe for them to be saved.

"One of the rides have sustained some sort of malfunction," Mr Fuller said.

"A number of the staff that were here today from Queensland Ambulance have been deeply affected by this terrible accident that has occurred today."

Dreamworld was closed down and evacuated an hour after the accident occurred at approximately 2.20pm.

The park will remain shut on Wednesday as Workplace Health and Safety Officials and police forensic crash investigators try to discover just how the incident happened, and it's unknown when it will reopen.

Park owner Ardent Leisure shares fell 7.8 per cent after the accident.

The Thunder River Rapids Ride is currently Australia's only river rapids ride and opened in December 1986.

Dreamworld's Rocky Hollow Log Ride was shut down in April this year when a man fell from the ride and nearly drowned.

*Readers seeking support can contact BeyondBlue on 1300 224 636 or Lifeline on 13 11 14