well i recently got back from driving to melb to qld, through inland country NSW (newell hwy) and all of that.
i was driving the old box which runs regular, so can't say I was checking every stop, but most of the larger ones along the way had Premium of some type (maybe not optimax, but premium 96RON).
You may find it hard to find Shell servo's the whole way, but if you pace it properly you could probably get away with just Shell. Through NSW at least, it may be 100km between fuel stops.
If you go along the Hume, or more towards the coast, you should have even less problems. Most of the big roadhouse stops have had Optimax for many years now.