Thanks for the replies.
The scenario I’m imagining is one where the Dr / specialist owns an aircraft (but is not a pilot).
They of course pick up all the costs for running the aircraft, but they engage a CPL to fly them to their country appointments a couple of times a month.
The Dr pays the CPL for their time.
In that situation it would strike me as very bizarre indeed if CASA came along and said it needs to be done under an AOC.