The investigation had been deferred on three separate occasions.

Max Quartermain, the 63-year-old owner and pilot for charter company Corporate and Leisure Aviation faced referral to the Civil Aviation Safety Authority and possible suspension of his aviation licence over a "near collision" with another plane on Mount Hotham in September 2015.

But the draft investigation report has been delayed for more than eight months because of "competing priorities and workload of the investigator in charge", according to a recent update from the Australian Transport Safety Bureau.

"Completion of the draft investigation report has been further delayed by the involvement of the investigator in charge on other aviation safety investigations and tasks."

The Federal government department now expects the report to be finalised by May 2017. 

The investigation was launched after Mr Quartermain was at the helm of a Beechcraft B200 King Air that took 87 minutes to reach Mount Hotham Airport – a flight that usually takes 38 minutes.

Mr Quartermain was transporting Audi customers to an exclusive event at the alpine resort, when he came within 1.8 kilometres horizontally and 90 metres vertically to a plane from Sydney that was ferrying passengers to the same function.

In bad conditions, Mr Quartermain was forced to abandon his first landing attempt.

The pilot of the plane from Sydney claimed Mr Quartermain's actions were "unsafe" in an incident report to the Australian Transport Safety Bureau. 

"If this event did result in a mid-air collision, two aircraft would have been destroyed and 18 people would have been killed," the Sydney pilot claimed in his report.

At one point, it was alleged that Mr Quartermain radioed to say he was 10 nautical miles (19.5 kilometres) west of Mount Hotham, before correcting himself to say he was actually 10 nautical miles east.

It is understood that some of the passengers on board the flight were left badly shaken and insisted Audi find a replacement pilot for the return to Melbourne.

The Australian Transport Safety Bureau confirmed it would investigate the "navigation and autopilot system" fitted to Mr Quartermain's plane, which was a different craft to the one destroyed in the deadly crash at Essendon Airport on Tuesday.

According to the Corporate and Leisure Aviation website, Mr Quartermain was the holder and operator of an Air Operations Certificate for more than 38 years and had an "impeccable safety record".

The company had arranged premium charter flights to corporate functions and sporting events for more than 25 years.

A spokesman for the company declined to make any comment on Tuesday.

Mr Quartermain gave fellow pilot Peter Mathew his start in the industry, flying fishing charters out to Flinders Island. 

Mr Mathew paid tribute to his friend, who he described as an extremely safe and reliable pilot.

"He gave me my start as a young pilot. He was a true gentleman. He was very honest, a very reliable person. Nothing ever got him down".

Bart Crawley flew often with Mr Quartermain. He described his friend as "incredibly thoughtful, caring, supportive, and very responsible. You certainly always felt safe when he was behind the wheel".

But several pilots expressed concern about the decision by Mr Quartermain to return to Essendon Airport just seconds after take-off.

One aviation expert said turning the plane around would have resulted in a massive loss of power, while pilots are instructed to search for clear space and attempt an emergency landing in the event of mechanical failure.