Two helicopters collided in mid-air, killing four people Monday at Australia's Gold Coast tourism hotspot, officials said, with images showing one of the aircraft's rotors lying on a sandbar.

Three other people were in critical condition, police said, describing how bystanders had rushed to haul passengers from an upside-down helicopter.

One helicopter had flipped over on the exposed sandbank a few feet from the shore, images on public broadcaster ABC showed.

The aircraft was shorn of its rotors, which lay a short distance away.

The other helicopter appeared to be largely intact at the accident scene, which is near the Gold Coast's popular Sea World marine theme park.

A bright yellow rescue helicopter had landed on the sand nearby, its rotors spinning, images of the aftermath showed, with scores of rescuers deployed around the area.

Several small police and rescue boats were moored on the exposed sandbank, which was strewn with bits of the destroyed helicopter.

"Those two aircraft, when collided, have crash landed, and landed, on the sandbank just out from Sea World resort," Queensland police service acting inspector Gary Worrell told a news conference at the scene.

"As a result of that, four people have lost their lives today," he said. "Three others are critical in hospital."

- Investigation launched -

One of the helicopters appeared to have been taking off as the other was landing, Worrell said, adding that the facts were yet to be determined by air accident investigators.

The helicopter that landed safely had lost its windscreen, the police office said. "The other airframe has crashed and it was upside down."

"Members of the public and police tried to remove the people and they commenced first aid to try and get those people to safety (from) the airframe that was upside down."

Access to the scene was complicated by the sandbank's location, Worrell said.

"I would like to thank all those that attended and made (it) possible to get those people to emergency care like they did."

Australia's transport safety bureau said it had already launched a probe into the mid-air collision.

Investigators would examine the wreckage and map the site, as well as recover components for examination, it said in a statement.

The bureau appealed to anyone who saw the helicopters in flight or had any footage to make contact.

A preliminary report was expected in between six to eight weeks.