Not sure of the applicable amendment you are referring with AIP as i use Jepps.
You have never been able to use QNH source from a METAR for the extra 100 ft on mda's.
However In Jepp Terminal 5.3 it advises on what can be used to reduce the mda by 100ft. Briefly the sources are ATC, ATIS, AWIB and CASA approved met observers. Additionally in Jepp Meteorology 4.3.6 it also advises that QNH derived from AWS is an acceptable source of actual QNH for reducing MDA. The QNH is valid for 15 mins from the time of receipt.
It is nonetheless confusing as an AWS observation appears with the title METAR