ATSB
Because the ATSB's primary role is only to investigate aircraft incidents and accidents (ie "after the event" stuff), with a secondary role of pilot education and safety prevention (mainly through the crash comics).
It's CASA's job to set the safety standards and to monitor regulatory compliance and to ensure the system is implemented safely . An important point to realise here is that CASA is not required to conduct a formal safety analysis to determine if the whole NAS concept is inherently safe or not because, under the ICAO guidelines, there is no requirement to undertake a full safety analysis if the proposed airspace system is a "tried and tested" system already in operation in another part of the world.
So CASA's safety role is restricted (unfortunately) to only conducting safety analyses on the implementation of NAS not on the merits or demerits of the concept itself.