If your Plan’s in NSW and it was Registered after July 1974, then the balcony door is Common Property and YES you should obtain the prior consent of the Owners Corporation (O/C) before fitting your cat door to its door, and conversely if your Plan is pre-1974 then the door’s yours and you can go ahead change it in the way that you propose provided the work’s done by a properly qualified person.
Strictly speaking, where required the above consent should be granted via a General Meeting, but if the timing of that doesn’t meet yours, then as the works comprise a relatively minor change to the Common Property, perhaps put the proposal in writing to your Executive Committee together with details of the tradesperson who’s doing the work, a copy of their liability insurance, and an undertaking to thereafter accept responsibility for the repair and maintenance of the modified balcony door and to restore it to its original state prior to any future sale by you of the Unit.
That would satisfy me, but then I don’t know how your O/C operates.