The collision occurred at around 14:00 local time, when the park would have been filled with families enjoying the school summer holidays.

One helicopter carrying seven people was taking off, and the other helicopter with six people on board was landing at the time of the crash.

Australian Transport Safety Bureau (ATSB) Chief Commissioner Angus Mitchell said that the incident is a "tragic start to the new year".

He explained that the main rotors of the helicopter taking off collided with the landing helicopter around 200m in the air, causing considerable damage to the front section. During the process, the main rotor blade and gearbox detached from the main body of the aircraft.

The helicopter coming in to land "remarkably" landed upright, despite considerable damage to the front left section of the aircraft, where the pilot was seated.

Members of the public were among those first on the scene, with a sandbank making it difficult for emergency service workers to access the site of the crash.

"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 the Queensland Ambulance Service, said 13 people had been on the two helicopters.

Three critically injured people were suffering from "multi-system trauma," while six other people were taken to the hospital with minor injuries, primarily from the impact of the glass.

Sea World Helicopters, a separate company from the theme park, expressed its condolences, saying it was "devastated" by what had happened and was cooperating in an investigation opened by authorities exploring the cause of the crash.

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

A witness told Melbourne radio station 3AW that there was a "massive, massive bang" and staff at the theme park moved swiftly to close off areas closest to the crash.

"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".

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

What is the process now?

Mitchell explained that the ATSB will continue to gather all possible intelligence, including camera footage taken by members of the public and CCTV from nearby buildings.

"What we need to know now is what was going on inside those two cockpits at the time," he said. We know takeoff and landing are critical phases of any flight where the cognitive workload of pilots is at its greatest.

"We will get what we can from those two helicopters to try and get a better picture of what happened and ultimately look at the processes that were in place that are designed to protect helicopters in this situation.

"We know that it is a busy time of the year and helicopters are in and out of the Sea World helipad constantly throughout the day. We will be looking at multiple lines of inquiry here around the procedures that are in place and what potentially helped to contribute to such a tragic incident."

Sky News