The plane crash at a local Melbourne shopping mall killing 5 person onboard, is still under investigation but another investigation has also started about the pilot flying the plane.

Experienced pilot Max Quartermain was accused by a fellow pilot of nearly causing the death of 18 people aboard the two planes, which had travelled to Mt Hotham, high in the Victorian alps, for a corporate ski event in September 2015.

The Australian Transport Safety Bureau investigation remains ongoing after a series of administrative delays. Its investigators uncovered evidence that Quartermain, after experiencing problems with his GPS equipment in cloudy skies, steered his plane into the flight path of other aircraft trying to land.

The plane flown by Quartermain came within 90 vertical metres of another craft, the investigation was told.

The Australian can reveal that Quartermain last year failed a Civil Aviation Safety Authority proficiency test before passing it on a second attempt. He was pilot of 38-years experience and on the website promoting his charter company, claimed to have an impeccable safety record.

As a pilot over the age of 60, Quartermain was required to sit a proficiency test every six months.

The ATSB investigation is due to publish it final report into the incident in May.