Workers at Dreamworld endured a "culture of fear and intimidation" and issues around adequate staffing of rides, according to the Australian Workers' Union.
AWU Queensland secretary Ben Swan said that in the Dreamworld environment - with "a lot of heavy plant machinery, water, hazardous obstacles and an interface with the general public" - you needed enough staff to watch the rides "like hawks".
Mr Swan said the union became aware of the culture of fear while investigating incidents at the theme park in November 2014.
"Once you initiate a process like that, other things start coming out of the woodwork," he told Radio 2UE. "We initially had some issues about the level of manning.
"What has come out is that people have a perception about a culture of fear and intimidation within the workplace which prevents - acts as an inhibitor - to the exchange of important bits of information which actually might affect the way things are operated, the way things are maintained and that affects safety."
His warning comes after 32-year-old Kate Goodchild, from Canberra, her brother Luke Dorsett, his partner Roozi Araghi and 42-year-old Sydney-based New Zealander Cindy Low were killed when their raft overturned on a conveyor-belt. Ms Goodchild's 13-year-old daughter Ebony and Ms Low's 10-year-old son Kieran were miraculously thrown clear.
In the wake of the tragedy, 143 pages of documents have emerged that reveal Dreamworld was told about rust, cracks and issues with rides in inspections between 2010 and 2016.
The theme park, owned by Ardent Leisure, was even warned in one report released to AWU that the deadly Thunder River Rapids ride was "not fit for service."
Mr Swan described the reported operation of the emergency button on the Thunder River Rapids ride by an 18-year-old woman on her first day at Dreamworld as "concerning".
Asked why he believed the park's management had blocked the publication of a document regarding safety and manning issues, Mr Swan said he couldn't "speculate the motivation".
Ardent this morning is holding its annual general meeting to decide executive bonuses.
"To be fair, I don't think it's unusual for companies not to want to keep information relating to their internal processes from public gaze," he said.
"I don't make any comment about that one way or other than I think the protracted nature of that objection hasn't been productive.
"If they had chosen a path way of engaging with the union and our members, people who operate these rides and maintain these rides a little sooner we could potentially be in a different space.
"I don't elevate it any higher than that, but it's unfortunate in the way it's been resisted ... and continues to be resisted."
Mr Swan said the union had been "absolutely" raising concerns with Dreamworld for some time about safety.
"Workers who operate these rides have legitimate interests and as their industrial representative my union has a legitimate right to be at the table talking with the company about this," he said.
He added that the union's concerns resulting from the incidents in November 2014 began with "operator numbers, manning on particular pieces of equipment" but expanded thereafter.
"In any environment where you have a lot of heavy plant machinery and equipment, in this case rides, and you have water involved, hazardous obstacles and you've got an interface with the general public, that elevates things," he said.
"You want to make sure you have a fairly robust operating environment.
"That means you need to have a certain cohort of people in your employ that are actually monitoring this stuff like hawks."
Mr Swan said he hoped Dreamworld would in the future be able to resume normal operations, for the benefit of staff working at the park and the public.
Dreamworld will partially reopen tomorrow for a memorial day, with all proceeds going to the Australian Red Cross.
The company said in a strongly worded statement this morning: "Dreamworld has reached out to the families through Queensland Police to offer its unwavering support.
"Our deepest sympathies and heartfelt thoughts remain with the families and loved ones of the victims, the first responders on the scene, guests who were at the park and Dreamworld staff.
"Park safety is our priority. Dreamworld would like to assure the public and park guests that at the time of the incident the park was fully compliant with all required safety certifications.
"We can assure you, the entire Dreamworld team is committed to working closely with the
investigating authorities on all matters of safety, process and procedures relating to the incident.
"We support the Mayor and the Premier's sentiment that we can leave no stone unturned.
Safety of our guests, staff and wildlife is a core value of our business.
"We welcomed almost 1.8 million people to Dreamworld last financial year and prior to Tuesday there has never been a death at the park due to a ride incident, despite Dreamworld hosting more than 30 million visitors since it opened in 1981.
"This is largely due to our robust policies and procedures and our 1000 plus employees who are totally committed to guest services and guest safety. The importance of safety is inherent in all our operations and decisions at every level of the business.
"All our procedures and systems are constantly benchmarked against international best practice and ride manufacturer specifications.
"Our rides and slides are checked and tested by our experienced team before the park opens every day.
"If it's not tested, it doesn't open."