Welcome to the IREX question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
Individual operators can be approved by CASA/BOM to give valid local QNH on a company frequency. Although it's hard to find a specific AIP reference concerning the use of AWIS delivered by mobile phone, there are some common sense interpretations. For example, when it comes to alternate planning, a shaded box in DAP says you can subtract 100ft from the alternate minimum.
However, since the decision concerning the provision of an alternate must be made before take-off, how can you be certain that you will have mobile phone coverage when you arrive at the destination? It's too late then to discover that you should have carried an alternate.
As far as the actual instrument approach is concerned, an AWIS delivered by mobile phone would be fine because if you don't have coverage, you don't subtract 100ft from MDA.