No-ones fault, just a stupid system, just asking for same to happen again, especially when everyone is inexperienced, busy or distracted....
Agree.
If you bring incoming traffic into a higher circuit above outgoing traffic, the conflict that caused this accident is removed. Incoming 1500 AGL, outgoing 1000 AGL. Descent to 1000 AGL is allowed by the controller when he sees space. It's done at other Class D, why not at Moorabbin? Perhaps there is a reason, I'd like to hear it.
(I note in ERSA departures are supposed to climb to 2000 or higher, to separate them from arrivals. This doesn't remove the conflict when arrivals at 1000 AGL join a busy circuit at the same altitude, which happened here).