It would be interesting to find out accurate stats on the number of times Centre tells an aircraft their CTAF call was on Area. If what I hear is indicative, I reckon the number would be 'lots'. At least in those cases a third party has let the aircraft know - or at least tried to let it know.
One tricky issue that I fear could loom large in the Caboolture tragedy is the 'regulatory status' of the local 'Operations Manual' and 'Local Traffic Regulation' in ERSA that says: "Carriage and use of radio is required by the AD OPR". I doubt whether the local 'Operations Manual' modifies or otherwise changes the legislated rules of the air, including the take-off/landing rules. And while the owner of a (sort of) flat piece of land can set just about any rules the owner likes about the use of that patch of land by aircraft - like you're not allowed to use it unless you carry and use a radio, I don't think that turns the area into a Mandatory Broadcast Area from a regulatory perspective.