It will depend a lot on what you intend to do with (or without) it. There have been discussions on this site as to the usefulness of the 'piece of paper'. Some intimated that grade viii was only a starting point as far as being a pro player is concerned - probably from a classical standpoint.
It also depends where you are in your playing standard, too. After grade viii, a lot of boards split stuff into performing or teaching - with quite different syllabi for each, not surprisingly.
There are myriads of players, professionals, in all sorts of genres, who don't have certificates to show their prowess. Their playing, thus reputations, does this for them.
Having said that, classical playing seems to attach a lot more importance to certification, rightly or wrongly. So, if the goal is to become a classical guitar player of note (sic) it quite possibly is the best way to go. It will depend on the board as to what you HAVE to get before moving on to the next step. Check out ABRSM, Trinity, LCM as some choices. Also one syllabus may suit you better, although actually taking the exam could be problematic in Australia - I don't know.