On 21 June 2012, after a review of the initial Airservices Australia report of the occurrence, the Australian Transport Safety Bureau (ATSB) initiated an investigation into a breakdown of separation (BOS) between two aircraft near Melbourne Airport, Victoria on 20 June 2012.

A preliminary examination of the occurrence showed that the BOS involved an Airbus A320 aircraft (A320) that was inbound to Melbourne Airport from the north and a Bombardier Inc BD-700 aircraft (Bombardier) that had departed from runway 34 at Melbourne and was tracking to the north-west. The ATSB also reviewed information from the involved Melbourne Approach Departures controller, the relevant air traffic control automatic voice recordings, weather information from the Bureau of Meteorology and the respective aircrafts flight tracks on Webtrack recordings (see Airservices WebTrak). 

The preliminary investigation determined that the controller had planned for a lesser rate of closure between the two aircraft than eventuated, with the actual rate of closure being affected by the ambient conditions and a slower acceleration by the Bombardier than anticipated by the controller. No avoiding action was necessary by either crew. The controller became aware of the infringement of the 3NM (5.6km) separation standard when advised by the crew of the A320 that they had received a Traffic Alert and Collision Avoidance System traffic advisory on passing behind the Bombardier. Shortly after, the lateral and vertical distance between the two aircraft increased and a separation standard was re-established.

The preliminary investigation showed that the potential for any systemic issues to have contributed to the occurrence was low and that it was unlikely that any safety issues would be identified through further investigation. On that basis, the ATSB decided to discontinue its investigation, as provided for in Section 21 (2) of the Transport Safety Investigation Act 2003.