Police are combing CCTV footage available for clues as to what caused the horror crash that killed four people at Dreamworld on Tuesday after a ride "malfunction".

Queensland Police Inspector Rod Reid said footage will be reviewed during the investigation that is expected to stretch overnight.

The victims are two women aged 42 and 32, and two men aged 38 and 35 and a forensic pathologist and cornor have inspected the scene.

"We will be here for several hours," he said. "It is a complex retrieval involving heavy equipment that will take several hours."

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The incident occurred on the water-based Thunder River Rapids ride about 2.20pm Queensland time.

Two riders were understood to be trapped by the underwater conveyor belt after the ride in front up-ended and two people were trapped in the raft. Two other riders were able to escape.

Reports suggest two of the victims were brother and sister although this is unconfirmed. It's also thought one of the women had children following eyewitness reports.

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Ejected, caught

Channel 7 breaking news report from Dreamworld where an accident on the Thunder River Rapid ride has resulted in death and injury

Gavin Fuller of Queensland Ambulance said two of the victims were "ejected" after a "malfunction" on the ride, while two others were caught inside.

The dead were aged from 32 to early 40s. Fuller said their injuries were so severe they were "incompatible with life".

Police refused to say if the victims were related or if any other relatives were on the ride at the time.

Witnesses said they saw a young girl crying who they believed was connected to the victims.

"We saw people hysterically crying, running around...One of the daughters said it was her family," a witness told Sky News.

Another ride at Dreamworld was shut down for a weekend in April after a man nearly drowned on it. Worksafe Queensland allowed it to reopen after an investigation.

Dreamworld CEO "deeply shocked and saddened"

Dreamworld chief executive Craig Davison confirmed that four adults died on the Thunder River Rapids ride at approximately 2.20pm.

He said the park is now closed and Dreamworld is "working to establish facts around the incident."

He said he was "deeply shocked and saddened", and "our hearts and thoughts go out to the families involved".

PM: "Very sad, tragic event"

Prime Minister Malcolm Turnbull offered his condolences in a statement.

"I'm very saddened to learn of the tragic accident at Dreamworld earlier today."

He said theme parks were a place for family fun and happiness, not tragedy.

"This is a very, very sad, tragic event. Our thoughts and prayers are with the families of those who have lost their lives."

Police unaware of earlier ride issues

Queensland police spokesman Tod Reid said the forensic crash unit and workplace health and safety were at the scene of the accident.

He said police were working with Dreamworld to determine what happened.

Reid said he wasn't aware of any earlier issues with the ride, and couldn't say if the four were related.

"There were heaps of people crying"

A witness told the Courier Mail that her sister and niece were on the ride.

"They are so traumatised, there is a woman hanging by her foot, crushed from the ride."

Another said the ride flipped at the very end of its circuit.

The same conveyor belt system used to operate the deadly Thunder River Rapids ride at Dreamworld was to blame for a near death on a similar ride at the park in April this year.

A man told the Courier Mail a girl was pulled from the ride just before it flipped. He said he thought there were four people on the ride at the time.

Another witness Leah Capes said she was waiting near the ride entrance when people came running from the rapids ride line.

"The kids and people in there were screaming.

"It all happened so quickly... there were heaps of people crying."

A Queensland Workplace Health and Safety spokesman said they had been informed of the incident.

Preliminary reports it may have been an industrial accident on a ride undergoing repair were false, he said.

"The scene I'm told was horrific"

Queensland Premier Annastacia Palaszczuk said the Workplace Health and Safety investigation could go into the early hours of the evening.

"The scene I'm told was horrific, it was a horrific scene. They are going to send counsellors there as soon as possible to help those people suffering enormous trauma.

"On behalf of all of Queensland I convey our thoughts and prayers to the two females and two males who are no longer with us."

"This is a very sad day for our city"

Gold Coast mayor Tom Tate issued a statement reflecting the community's shock at the news.

"This is a very sad day for our city," Tate said.

"Our thoughts are with the families of those affected - and the emergency staff in attendance."

Australia's only river rapids ride

The Thunder River Rapids Ride is currently Australia's only river rapids ride and opened on December 11, 1986, reports the Gold Coast Bulletin.

Riders board one of several six-person circular rafts.

The raft is dispatched and the riders travel back past the ride's queue and into a cave.

Upon exiting the cave, riders experience the main rapids section of the ride. This section runs alongside a large water catchment which powers both the Thunder River Rapids Ride and the Rocky Hollow Log Ride.

Popular destination

A House of Travel spokesman said the Gold Coast was "probably New Zealand's most popular short-haul family holiday destination". Nearly 190,000 New Zealanders travelled to the Gold Coast in the year up to March 2016.

The previous "serious incident" on the Log Ride nearly caused a man in his 30s to drown after the conveyor belt slipped and trapped the man.

Workplace Queensland and independent inspectors were forced to give the ride the all-clear before it was able to reopen again.​

MAJOR AMUSEMENT PARK ACCIDENTS IN AUSTRALIA SINCE 1968

• September 12, 2014 - Eight-year-old Adelene Leong dies after being thrown from a high-speed ride at the Royal Adelaide Show.
• May 18, 2013 - Five-year-old boy seriously injured after being flung from the Frizbee ride at a school fete in Highfields, Queensland.
• March 11, 2001 - Eight-year-old girl dies and 11 people injured when an inflatable carriage breaks free from a ride at a carnival in Kapunda, South Australia.
• Sept 2, 2000 - Thirty-seven people hurt when the Spin Dragon ride collapses and drops a 4.3 tonne gondola onto queuing riders at the Royal Adelaide Show.
• Feb. 22, 1997 - 11-year-old girl dies and two boys injured when a carriage on the Octopus ride breaks free and falls to the ground at the Rylstone Show near Mudgee, NSW.
• June 9, 1979 - Six children and one adult die when Sydney's Luna Park's Ghost Train catches fire. This occurred two months after 13 people were injured on the Big Dipper when a steel runner came loose on a rollercoaster train on April 16.
• October 24 1968 - High-wire stuntman Adrian Labans, 44, falls to his death during a highwire act at the Royal Hobart Show.

- With AAP