The only help that I can be is, if you are after a regulation the AIP is USUALLY no help but, CAR 82A says
" casa may give direction to words or phrases used and this will be in the form of the AIP or NOTAMS" or close to that anyway as I can't paste it. This now makes not adhering to the (I us jepps) communication section, or AIP, a breach of 82A.
Then in the AIP somewhere (AU 915.6.1.5 for jepps) in the communicationssection that the "following information whilst primarily for controlled airspace should use these phrases unless specific class G procedures are shown."
Then 6.3.3 goes on to say that " reported level figures must be preceded by flight level when referenced to standard pressure and feet when referenced to QNH"
That now making using feet with anything apart from QNH In breach of 82A.
Also there is not reference to AGL that I've seen in my late night reeding of it so therefore, wouldn't be directed by casa.
I apologise for the jepps references but I don't even know how to spell AIP let alone cross reference them.
So it's kind of an asre about way of getting there but if they were to prove that wrong they have more time and effort to put into it then I have. But with that now out for all of the Internet to see I'll imagine someone will come up with something, and my bet is the legal definition of should.