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A helicopter pilot said he did not hear a vital radio call shortly before a deadly mid-air collision in Australia, according to an interim report.

The report also said the pilots may not have been able to see each other.

The two helicopters collided off the Gold Coast in January, killing four people and injuring several.

One helicopter, which had just taken off, ended up crashing. The other, which was returning, managed to land with heavy damage.

The aircraft were used for short sightseeing trips by Sea World Helicopters.

The report released by the Australian Transport Safety Bureau (ATSB) details the calls made by Michael James, who was piloting the returning helicopter.

He said he saw passengers boarding the other helicopter as it was preparing to depart. He did not see it take off, but he thought it would pass behind him.

He told investigators he "did not recall the pilot [of the other helicopter] making a standard taxiing call, thereby announcing their intentions to depart", according to the report.

The ATSB said this did not mean a taxiing call was not made, and that they would carry out a detailed analysis of radio activity at the time of the crash.

The pilot of the departing helicopter, 40-year-old Ashley Jenkinson, was killed in the crash.

Footage later emerged of a passenger trying to warn a pilot that another aircraft was approaching.

But the report said this did not mean that the two pilots had seen each other's helicopters.

The two aircraft collided at a height of 130ft (39 metres), with the rotor blades of Mr Jenkinson's helicopter smashing into the cabin of the other, the report said.

Mr Jenkinson's helicopter broke apart and crashed into shallow water just off the shore, killing him and three passengers.

The ATSB said the "complex and comprehensive" investigation will scrutinise operating procedures to identify any underlying safety issues.

The helicopters were flying in non-controlled airspace, where pilots use a common radio frequency to communicate with other aircraft, the ATSB said.

The victims included a British couple, 65-year-old Ron and 57-year-old Diane Hughes from Cheshire, who were holidaying in Queensland at the time, police said.

Australian woman Vanessa Tadros, 36, also died in the crash, and her 10-year-old son Nicholas was among the seriously injured survivors.