Management at Dreamworld knew of safety problems affecting its rides for two years but did nothing, the Australian Workers' Union has claimed.

The union says it has repeatedly raised concerns since 2015 and as recently as three weeks before the accident that claimed the lives of four people at the Gold Coast park.

AWU Queensland secretary Ben Swan said "categorically" the accident will "not come as a surprise" to Dreamworld management.

"This wasn't just a single event of raising an issue, this has been ongoing for 18 months or more."

Brother and sister Luke Dorsett, 35, and Kate Goodchild, 32, Mr Dorsett's partner Roozi Araghi, 38, and Sydney mother Cindy Low, 42, died when their raft flipped on the Thunder River Rapids ride on Tuesday afternoon.

'We've met resistance'

Mr Swan said he was not aware of specific concerns with the Thunder River Rapids ride but that the union had made direct representations to the company about "operational and mechanical faults" affecting other rides.

However, he said the company resisted action to address the issues.

In April last year, the company opposed the union's right-to-information requests around safety issues.

Management visited the union's offices to discuss the concerns but the union said no further action was taken.

"We've met resistance which has not been constructive or positive," Mr Swan said.

"We don't do these things lightly - we do this because the lives of our members and of the general public can be put at risk... when workers come to their union repeatedly [with concerns] - they're at the coal face, they see what happens."

Dreamworld owner Ardent Leisure, whose shares fell up to 22 per cent on Wednesday, is now co-operating with a coronial investigation into the deaths while the park remains closed.

Ardent released a statement on Wednesday defending its safety record.

Fines of up to $3 million

The accident could see Dreamworld management, Ardent chief executive Deborah Thomas and board members fined hundreds of thousands of dollars in fines and even prison if they are found to have been negligent in their duties.

Leading expert in workplace safety, Clyde & Co partner Michael Tooma said there was no limit to how far up liability could go under occupational health and safety legislation, which covers employees and members of the public.

The legislation, a criminal statute, imposes fines of up to $3 million on companies for the most serious cases.

Board directors, senior management and other officers face fines of up to $600,000 or fives years in prison if they are found to be reckless in failing to exercise their "due diligence" to ensure no one is put at risk.

"If someone is aware of something and ignores it then that is exactly the thing the legislation contemplates when it's talking about reckless conduct," Mr Tooma said.

Incidents that involve multiple deaths and occur in high-risk environments, such as amusement parks, are likely to be considered in the most serious category, he said.

Ardent is also vulnerable to civil claims of negligence, where damages are not capped and the standard of proof is lower.