DJP
I don't see anything that exempts things already covered by an STC. I'm thinking mainly about things like the JPI OAT sensor, or maybe an AoA sensor (which are becoming more popular). Our aeroplane has the LoPresti hub caps fitted, that should now require a flight test (despite its STC) and ditto LoPresti spats, gear lobes, and other fairings.
Also the AC talks about replacing items (substitution of a part or appliance), which theoretically says if you replace a TXP antennae with an ADS-B one, or a GPS antennae with a WAAS one of an ELT antenna with a 406 one then you need a flight test. Technically, its saying that if you replace a landing light you need a test flight.
"anything attached to external surfaces..." also catches the press studs for aircraft covers and the unique Australian requirement for grounding tabs adjacent to fuel caps. It probably also covers fitment or replacement of static wicks. Taken to the extreme, this covers self adhesive aircraft registration or other decals.
I also think flight test requires some definition. Is it a circuit? Is it a local flight with clean & dirty stalls? Does it include VNE dives? Does it require a flutter test regime? What constitutes a flight test pass? Flight testing is a qualitative thing, so it needs guidelines. Does the flight tester need to fly the aircraft before and after to look for differences? Many, many GA aircraft are mis-rigged. Should this be checked & rectified first so the pilot is test flying the mod on an aircraft rigged within the TC spec?
To me there needs to be something that allows discretion. Component replacement should be exempted, fitment of items according to an STC should be exempted (unless required by the STC) and minor items that don't materially affect the airflow or are within the boundary layer should be exempted.
The trouble is that CASA has a bee in its bonnet specifically about cameras and they are getting small enough to fall within most exemptions you might draft. There are already cameras smaller than GoPro's.