The devastated mother of two victims of the Dreamworld theme park tragedy says her family has been "wiped out" following their deaths on the Thunder River Rapids ride.

Kim Dorsett's two adult children, Kate Goodchild, 32, and Luke Dorsett, 35, from Canberra, were among four people killed when their raft flipped on the popular ride about 2.20pm on Tuesday.

Also killed was Mr Dorsett's partner, Roozbeh Araghi, 38, and a 42-year-old woman from Sydney. That woman's 10-year-old son was one of two children who were also on the ride and were injured when they were thrown from the raft. Both are recovering in hospital.

Ms Dorsett, who was holidaying in Queensland from Canberra with her children, told News Corp Australia she was "absolutely devastated" at the deaths of her "wonderful" children.

She said they had decided to visit Dreamworld, Australia's largest theme park, after extending their holiday.

"I have three children and have lost two of them today – my whole family has been wiped out," she said.

"They really were just wonderful people – that would be the best way to describe them. They were kind, loving souls that would honestly do anything for anyone."

Ms Goodchild had two daughters, aged 12 and eight months, and the older girl is believed to have been at the theme park when the tragedy occurred.

"Like me, she [the older daughter] is still coming to terms with it all but she is completely devastated – she is blaming herself for what has happened," Ms Dorsett said.

"She has had a truly terrible day, she is going through unimaginable pain at the moment. I'm at a loss as to how to deal with this now – I woke up this morning with three children and tomorrow I am only going to have one left.

"I have two granddaughters ... and it truly breaks my heart to know that my eight-month-old is never going to get to know her mum."

The second woman killed in the accident was originally from Kawerau on New Zealand's North Island, but had lived in Sydney for at least the past 10 years with her Kiwi husband, according to the New Zealand Herald.

They had two children - a boy, 10, and a girl, six - and the family was on holidays in Queensland at the time. The woman's husband and daughter were not on the ride.

Dreamworld will remain closed on Wednesday as police examine how the accident occurred. Police have confirmed the tragedy was captured on CCTV camera, and detectives are reviewing that footage.

Thunder River Rapids is presently Australia's only river rapids ride and opened in December 1986. Passengers sit in circular six-person rafts as they travel along a fast-moving, man-made river at speeds of up to 45km/h. Dreamworld described Thunder River Rapids as a "moderate thrill" ride.

Early reports indicate a conveyor belt may have malfunctioned as the raft approached the end of the ride, ejecting two of the victims and pinning the other two either under water or in machinery.

Dreamworld visitors Kaylah Walker, 25, and her mother, Lisa, said the same ride was not working earlier on Tuesday.

"It had broken down and we went back a couple of times to this particular ride," she said.

"We were standing on the bridge watching and the water had stopped."

Dreamworld chief executive Craig Davidson said the thoughts of the park were with the families and loved ones of those killed.

"We're all deeply shocked and saddened by this," Mr Davidson said.

"We will work closely with police and emergency authorities in this matter."

Prime Minister Malcolm Turnbull said there would be a thorough investigation.

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

As night fell on the park on Tuesday, tributes were being left for the victims of the tragedy.

One local woman left four candles, a small bunch of Gardenia flowers and a note saying: "Always in our hearts. RIP always."

Queensland Premier Annastacia Palaszczuk said it was a "dark day" in the state's history.

Ms Palaszczuk could not confirm the ages of the children who were injured on the ride, but said both were in the Gold Coast University Hospital.