What administrative decision do you say has been made that is reviewable by the AAT? Please provide one sentence stating the precise decision that you say CASA has made, one sentence stating the precise provision under the civil aviation rules that you say CASA made that decision and one sentence stating the precise provision of either the civil aviation rules or the AAT Act that gives the AAT jurisdiction to review that decision.
"CASA decided to [e.g. refuse to consider my ... / refuse to grant me...]. CASA made that decision under [e.g. CASR x]. CASR y says that decisions under CASR x are reviewable by the AAT."
If CASA has made a decision reviewable by the AAT in your case, you are entitled to reasons for that decision and CASA would by now have provided you with a thing called a 'section 37 statement' (which is a reference to a section of the AAT Act). Have you been given a 'section 37 statement'? If not, that's a pretty good indication that no decision reviewable by the AAT has been made.
I should stress that I'm on your 'side', in the sense that I despair when I see folks expending so much energy running 'vibe of the thing' arguments and making bush lawyer assertions on PPRuNe as if that advances their interests or bothers CASA in any way. But I'm reminded of the advice of a wise mentor of mine: Leaddie, you can lead a horse to water but you don't have to suck through its *rse to make it drink.
Good luck.