CASA did no have to pull Ansetts AOC: they were complicit in Ansett;s demise when they failed to properly and correctly undertake their obligatory oversight of Ansett, in particular Ansetts engineering responsibilities.
I.e. it is CASA's responsibility to ensure manufacturers modifications are carried out where in the case of the Ansett Boeing 767 they did not ensure Ansett carried out the rear pressure bulkhead modification; only when Boeing checked to see if this was done by the due date did the whatsit hit the fan when Boeing was obliged to tell
Ansett and CASA whereupon CASA had no option but to ground the aircraft. QED.
(Boeing was obliged i,e Boeing had to protect themselves)