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.
Having never flown into Moorabbin, I'm a bit shocked that this isn't already the case to be honest, as it already is elsewhere. Sure, in Class D pilots are responsible for their own separation, but that doesn't change the fact that any system that practically induces conflict is extremely problematic!
(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).
Yep... there's still the conflict with the departing aircraft on climb and arriving aircraft on descent - especially when you've got a tired 30 year old box struggling to make 500fpm ROC. Ridiculous!