Paramedics, fire crews and police were on scene at the Coomera theme park near the Gold Coast responding to reports received just after 2pm that a number of people had been injured by a conveyor belt.

Paramedics said two people had been thrown from the raft and two people were trapped inside.

Queensland Ambulance Service officer Gavin Fuller said the people were aged from 32 through to early 40s.

He said park staff attempted first aid but weren't able to save them.

Dreamworld chief executive Craig Davidson said the park had been closed.

"We are deeply shocked and saddened by this and our hearts and our thoughts go to the families involved and their loved ones," he said.

One woman who spoke to media at the theme park said a young girl was crying and being comforted at the ride.

"We think it was her mum that was involved," the woman said.

"There was just police and lifeguards everywhere.

"It happened on the Rapids ride. I spoke to a guy and he said he looked behind and the whole thing flipped over."

Another witness said she believed three adults involved in the accident were from the same extended family.

The theme park had earlier released a statement as reports conflicted over the extent of the tragedy.

"Dreamworld is working as quickly as possible to establish the facts around the incident and is working closely with emergency authorities and police to do this," the theme park said in a statement.

"Dreamworld's focus and priority is with the families of those involved in this tragedy and will be providing an update to the public as soon as information becomes available."

A concerned mother whose daughter was at Dreamworld at the time of the incident voiced relief to Facebook after she was told those involved were not children.

"Ok I'm here at park and have been informed the ppl (sic) in the accident are over 30 now to find my girl they are evacuating park now thanks all," she wrote.

Gold Coast mayor Tom Tate said it was a "very sad day for our city".

"Our thoughts are with the families of those affected – and the emergency staff in attendance," he said.

"I urge everyone to show the respect needed as the relevant authorities undertake investigations in to what has occurred.

"A truly sad day for all."

The incident comes about six months after a near-drowning on Dreamworld's popular log ride.

A man was on the log ride when he fell out in April.

He was treated by the theme park's medical staff after he swallowed water and suffered head cuts before paramedics arrived.