To be fair self regulation does not mean it has to have an investigation arm when an independent transport safety investigation bureau already exists. CASA defers to the ATSB or Police for investigations beyond a certain scope and as mentioned before having the facilities and equipment to properly test and evaluate materials and data would not be feasible for such operations.
On the other hand, what more is there to learn here, the aircraft either met unintentionally being unaware of each other or intentionally where one party rammed the other. Without any proof of the later where the police and coroner need input it just comes down to two machines being in the same space/time point in unison and added to the statistics for an overall plan on how to stop it happening again. Spending hundreds of man hours on working out the exact points of impact and the pilots medical history probably wont reveal much else that they don't already know. And suggested outcomes that pilots already know about, communicating appropriately, looking out and planning to avoid glider areas if you can.