Two helicopters collided mid-air on Monday, killing four and seriously injuring three, south of Brisbane in Australia's Gold Coast tourist region.A mid-air collision between two helicopters in Australia has left at least four people dead and three others in critical condition, authorities said on Monday. Six other people were being treated for minor injuries, according to local media outlet Nine News Gold Coast. The collision happened around 2pm local time on the Gold Coast near Sea World water park, south of Brisbane. Television images show the rotors of a device lying on a sandbar a few meters from the shore. The other helicopter appeared largely intact at the crash site."This is a business associated with Sea World with an operator within a very large corporate circle," Neil Hansford, president of Strategic Aviation Solutions, told Sky News Australia. After the accident, a rescue helicopter landed on the sand, while dozens of rescuers spread out on the ground. Several police and rescue vessels stood ready to intervene near the sandbar strewn with debris from the two helicopters.The Australian Transport Safety Board announced the opening of an investigation into the accident, presented as a "mid-air collision". Investigators were expected on site to examine the wreckage, map the site and recover investigative material, the office said in a statement.