The emergency stop switch on the Thunder River Rapids ride was reportedly being controlled by an 18-year-old woman on her first day on the job.

According to 7 News, a failed water pump caused a raft to become lodged at the top of a conveyor belt at the end of the ride in the Gold Coast theme park on Tuesday.

A separate raft behind it continued to be pulled along by the belt, hitting the stuck raft and flipping upside down as the machinery continued to churn forwards, the Daily Mail reports.

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Investigators say four adults were crushed and drowned when they were ejected from the ride, which had reportedly malfunctioned twice earlier during the day.

It comes as a former Dreamworld employee claimed that the 'family' ride has been suffering problems for nearly 30 years and has a "potentially fatal flaw."

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Jon Armstrong told news.com.au he was employed at the Gold Coast theme park on a casual basis for six months in 1987, the year after the family ride was first opened.

While working as ride operator, the 51-year-old from southern Queensland said he had watched on in horror as he witnessed a "similar problem" unfold.

Queensland Emergency service personnel at the scene of the accident. Photo / AAP
Queensland Emergency service personnel at the scene of the accident. Photo / AAP

Armstrong claimed that while he was on "start-up duties" for the ride before the park opened - one raft that was still attached to its rope mooring got stuck and caused three other rafts to flip over, and four more to mount those upended rafts.

"I was unaware this particular morning that one raft had two mooring lines attached by the overnight maintenance crew, the second rope being hidden on the far side of the raft and submerged," he explained.

"When the ride started, this raft stuck and caused three other rafts to flip on the conveyor and four more to 'mount' the flipped rafts.

"It's lucky there was no one on board because if there had been, their body parts would have been pushed under the water ... and at that part, the water's deep."

Investigators believe a six-person raft flipped, crushing and drowning two Canberra men, aged 38 and 35, and two women, aged 42 and 32, about 2.20pm on Tuesday.

New Zealand woman Cindy Low, 42, was among those killed.

Two children - a 10-year-old boy and a 12-year-old girl - were miraculously thrown from the boat and survived with only minor injuries.

The two kids, released from hospital Wednesday, were said to have seen their parents' deaths.

New Zealand woman Cindy Low was one of four people killed in the accident. Photo / Supplied
New Zealand woman Cindy Low was one of four people killed in the accident. Photo / Supplied

It was announced on Wednesday afternoon that the theme park would be re-opened on Friday morning at 11am, with entry proceeds going to Australian Red Cross.

"Activities will be limited to smaller rides, animal attractions, and the water park," Dreamworld said in a statement.

Listed under "family rides", the Thunder River Rapids ride, which has been open for 34 years, is compared to white water rafting.

"Travel down a foamy water track past the Gold Rush Country, speeding up to 45 km/h through the turbulent rapids," its website reads.

Children under the age of four must be accompanied by an adult.

Each circular raft seats up to six people.

Thrillseekers need to have "upper body control" and "must be able to maintain an upright seated posture", according to Dreamworld's Guests With Disabilities guidelines.

"Physical size must not reduce effectiveness of restraints," the guidelines read.

Reviewers online said the ride was "lame but fun" and "not a thrill ride but still cool".