What a report! I have been highly critical of recent ATSB reports, but this one seems to have done a very good, even handed job of going through all the issues. This accident highlights the severe problem of lack of training instrument aids in the Melbourne basin. If there had been a slot at Moorabbin, this accident would not have happened.
I need to re-read it but the swiss cheese holes seem to be:
While the pilots did everything that was required, although didn't initiate aircraft to aircraft comms for separation. They were both training, with the workload (and possibly belief that they were under IFR traffic separation conditions), its understandable. that they may have placed higher reliance than they should have for either another ATC warning or some clear direction.
The controller seems to have acted correctly in accordance with procedures, although I bet the poor guy lies awake wishing he'd done something extra at one of the STCA instances.
The ridiculousness of the CASA see & be seen concept.
Ambigous and poorly written CASA regulations.
Overloaded training aids in the Melbourne basin after the closure of Cowes, Wonthaggi
Maybe a failure of the AsA traffic system to determine STCA alarms according to flight plan rather than trajectory.