At least four people have been killed after two helicopters collided while flying over the Gold Coast in Australia on Monday.

Three people are in a critical condition, police said, while several more were also injured.

One of the helicopters was left upside-down on a stretch of sand, while the other helicopter was able to land successfully, albeit with a damaged windscreen.

Australia's transport safety bureau described the incident as a "mid-air collision" and has opened an investigation.

Queensland state 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 on Monday.

Passengers treated at the scene

The Queensland Ambulance Service said paramedics treated at least 13 people at the scene.

"Members of the public and police tried to remove the people and they commenced first aid and tried to get those people to safety from an airframe that was upside down," said Queensland acting police inspector Gary Worrell.

"(People on) Jet Skis, family boaters, ordinary members of the public rushed to assist these people."

Area near Sea World blocked off

Helicopter debris was strewn over Main Beach, a northern suburb of the Gold Coast that is home to the Sea World theme park and resort. It is unrelated to the American theme park of the same name.

Police cordoned off Sea World Drive near the site of the crash but the venue itself remained open.

According to ABC News, a Sea World logo was visible on the helicopter that landed successfully.

Sea World Helicopters operates as a separate entity from the theme park. In a statement, the company said it was cooperating with authorities but that it could not comment further on the incident.

"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," Sea World Helicopters said in the statement.

January is the peak holiday period in Australia, and when the Gold Coast is at its busiest. The city in Queensland is known for its beaches and is home to several large theme parks.