Former Dreamworld worker claims the raft ride had a "potentially fatal flaw".

A former Dreamworld employee has come forward to allege a “similar” incident of rafts flipping on the Thunder River Rapids ride was one he witnessed almost 30 years ago.

The claim comes just one day after a raft flipped on the ride and four adults were killed.

Jon Armstrong told News.com.au he worked as a ride operator on the ‘Thunder River Rapids’ in 1987 for around six months.

Armstrong said he witnessed a raft flip on the conveyor belt while it was still attached to its rope mooring during a routine check.

“I was on ‘start-up’ duties for the ride, which involved walking the river bed before the pump started, check the raft inflation pressure and ensuring all rafts were untied,” he said.

“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.”

“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."

The 51-year-old said the incident was considered among the team a hugely hazardous fault in design.

“Luckily we had not allowed guests in to ride yet, but those of us present at the time all agreed that this was a potentially fatal flaw," he said.

Armstrong said the incident led to an official investigation where precautions were tightened as a result.

Dreamworld was said to have installed cameras, moved the shutdown switch into the control room and added further checks to ensure rafts were untied.

The amusement park released a statement on Wednesday that said the ride had only recently passed its annual safety check.

“The Thunder River Rapids ride had successfully completed its annual mechanical and structural safety engineering inspection on 29 September 2016," the statement read.

"As per regulations, this safety audit was conducted by a specialist external engineering film. Details of this external audit will be provided to the Coroner and workplace safety investigators.”

Seven News reported the operator of the emergency stop switch was an 18-year-old woman completing her first shift.

The ride had reportedly malfunctioned twice that day but had not been shut down.

Dreamworld will re-open at 11am on Friday, October 28 to hold a memorial day honouring the families of the victims. Activities available at the park will be limited to smaller rides, animal attractions and the water park.

Channel Nine reports all proceeds from the day will go to the Australian Red Cross.