The bottom line is this.
Four people are dead because ATC could have separated them but did not.
Why not?
Because the level of service in any airspace is decided by the Office of Airspace Regulation which is required by Ministerial Direction to implement international best practice in airspace management.
VERY PLAINLY any system of airspace administration that is able to separate IFR aircaft but does not do so, in the full knowledge that those aircraft may be operating in conditions where they can not visually separate themselves is NOT applying any form of international best practice nor proven international systems.
No matter who overseas the OAR - it is their responsibility to carry out the Ministerial Direction and they have failed to do so.
Now let us make sure the Coroner and other investigative bodies are aware of the aspects of Aviation Legislation that you can bet nobody in CASA or AA will point them to.