Novicef,
Regarding the minima used at "main" aerodromes (like SYD, MEL, ADL etc) - on an Airservices approach plate, there is no shading on the minima, because tower is assumed to be active H24 and accurate QNH is assumed to ALWAYS be available.
For other aerodromes (which may not have accurate QNH sources) - you will find shading on the Airservices plate. With forecast TAF (Terminal) QNH, use the published minima. With an ACCURATE QNH (AWIS, Unicom, CAGRS etc), you may lower the minima by 100', and I believe Jepps show both minima on their plates.
Your question about "actual area QNH"... mmmn not sure what you mean. As far as I understood, there is no such thing. Maybe an ATC reader can help? I believe they just read the area QNH off the forecast.
You are right in saying that if using area QNH, one adds 50' to the minima.
So, for example at Dubbo, using the RW23 VOR, the Airservices minima is 1450 (for the S-I minima). As it is shaded, if you have an accurate QNH ('fresher' than 15mins old), you could use 1350'. On the other hand, if using area QNH (forecast/actual if there is such a thing), you would need to use 1500'.
I don't use them, but I imagine the Jepp plate for the same approach would show two minima - one of 1450' for forecast and one with 1350' for actual QNH.
Anyone else care to comment re : the idea of an "accurate area QNH"?
Cheers
VI