Four dead after two helicopters collide in mid-air near Sea World theme park in Australia

Two helicopters have collided near a marine theme park in the Australian state of Queensland, killing four people and leaving three others critically injured.

The mid-air collision, just outside Sea World on the Gold Coast, happened at around 2pm local time when the park would have been busy with families enjoying the summer school holidays.

Initial information suggested one of the helicopters had been taking off and the other had been landing when they collided, Queensland Police Acting Inspector Gary Worrell said.

He thanked members of the public who were among the first on the scene, adding that the location - a sandbank - had made access difficult.

"One airframe had the windscreen removed and it has landed safely on the island, the other airframe had crashed and it was upside down," Mr Worrell said.

"Members of the public and police tried to remove the people and they commenced first aid and tried to help these people to safety."

Janey Shearman, from Queensland Ambulance Service, said 13 people had been on the two helicopters.

The three people critically injured were suffering from "multi-system trauma", while six people with minor injuries - mainly from the impact of the glass - were also taken to hospital.

Footage of the crash showed a helicopter shortly after take-off being clipped by another aircraft flying over the water.

Sea World Helicopters, a separate company from the theme park, expressed its condolences and said it was co-operating with the authorities handling the crash investigation.

"We and the entire flying community are devastated by what has happened and our sincere condolences go to all those involved and especially the loved ones and family of the deceased," the statement said.

The company did not confirm if it operated one or both helicopters involved in the crash, saying it would not comment further because of the investigation.

A witness named John told Melbourne radio station 3AW that patrons at Sea World heard the crash, and staff moved swiftly to close off areas closest to the crash.

"There was a massive, massive bang," he said. "It was just huge.

"I'm not sure if it was the propellers or whatever hitting against each other. But there was this poor lady and her son near the helipad in tears."

Queensland premier Annastacia Palaszczuk said the accident was an "unthinkable tragedy".

"My deepest sympathies are with each of the families and everyone affected by this terrible accident," she said.

The Australian Transport Safety Bureau said an investigation into the cause of the crash was under way.