Tinstaafl - absolutely correct and Australia is not alone. Any country that constructs their procedures using ICAO PANSOPS will have similar statements in their AIP (or equivalent). What I find interesting, (as far as I can see anyway and happy for someone to direct me here) is there is no guidance provided in any official publication as to how to go about adjusting your MDA/DA. I have a couple of ways and I am sure others have their own.
Oktas8 - I have to confess to being a little confused. I presume the 300’ you talk about in your first point (a) is the normal ADH that one would select, regardless of conducting the flight under IFR or not? And I am unsure exactly what you mean with the comment in parenthesise. Could you explain a little more?
On your second point (b) I agree but with one addition (not really a correction); as you say the MAP is the least desirable option and one should consider themselves committed from the IAP. Therefore (and here is my addition) it would be unwise to even start the approach if visual reference is not assured at some margin above the normal MDA/DA. Put another way; you must establish an actual cloud base prior to approach commencement, this could be from ATC or a preceding aircraft having successfully completed the same approach.
You could take it a step further and say that descent below MSA (be it the holding MSA or route MSA) must not be considered unless a successful approach is assured, the exception being airspace with radar terrain services. I am not suggesting that you will be able to maintain every MSA as clearly any mountainous terrain MSA in a normally aspirated light twin with an engine out will be nigh impossible, but where practical.
Interested in more of this topic if anyone has any.
AML