Personally I would recommend consumption within six months to ensure the best quality and enjoyment. Beer which is past its optimum lifespan will usually have a distinctly unpleasant taste and smell, although it won't kill you (probably).
Keep in mind, though, that some specialty or "vintage" beers have extended shelf lives as they are meant to improve with age. Obviously these brews are not subject to the same recommendations that I have made in regard to general packaged beer, so check the particular manufacturer's guidelines for any beer of a "vintage" type.
So there you have it: beer is great! Opinions will always vary, but Australia does have some of the world's best beers.