A brother and sister were among four people killed on a ride at a theme park on Australia's Gold Coast.

Two men and two women in their 30s and 40s died in the accident on the Thunder River Rapids ride at Dreamworld - while police said two children thrown clear were lucky to be alive.

CCTV footage has shown that the ride was nearing its end when two rafts collided, Queensland Police Assistant Commissioner Brian Codd said.

"One has flipped backward and it has caught and tossed some of the people that were on the ride backward into the conveyor belt," he told reporters.

The theme park at Coomera has been closed and an investigation into the cause of the incident is under way.

Kim Dorsett, of Canberra, confirmed that two of the victims were her children - Kate Goodchild (32) and Luke Dorsett (35).

"I have three children and have lost two of them today - my whole family has been wiped out," she told The Courier-Mail newspaper.

She was on a family holiday with her children and Ms Goodchild's daughters from Canberra.

Police have declined to identify the other two victims, a 38-year-old man and a 42-year-old woman.

Media reports say the man was from Canberra and the woman was a New Zealand citizen who lived in Sydney.

The two children, a 10-year-old boy and a 12-year-old girl, who shared the raft with the victims were thrown clear and managed to escape, he said.

"In terms of how they escaped, maybe through the providence of God or somebody, but it seems from what I've seen almost a miracle that anybody came out of that," Mr Codd said.

"If we're going to be thankful for anything, I'm thankful for that."

The Dreamworld website describes the Thunder River Rapids as a "white water rafting" ride which travels at up to 45kph.

Visitors as young as two years old are allowed on the ride.

Dreamworld chief executive Craig Davidson said: "We are deeply shocked and saddened by this and our hearts and thoughts go to the families involved and their loved ones."

In a statement, the theme park said it was "working as quickly as possible to establish the facts around the incident".

It added: "Dreamworld's focus and priority is with the families of those involved in this tragedy and will be providing an update to the public as soon as information becomes available."

Australian Prime Minister Malcolm Turnbull said it was a "very, very, sad, tragic event".

He said: "Theme parks are a place for family fun and happiness, not tragedy."