It's a step in the right direction, but not perfect; eventually they'll come to realise that they need MA15+ and R18+ ratings at the same time.
At this stage, you won't get R18+ games being shoehorned into a lesser catagory that they don't belong to, exposing younger audiences to inappropiate content. Yes, it's a pain that MA15 games are now placed further out of reach of older teenagers, but I don't see why Austalia should be maintaining its schism of what happens to adult games (no, we don't like them, but they're welcome if you trim a little bit off).
The truely bad thing that could happen (from a protective standpoint, great for the 15-17s you mentioned) would be if the ratings board started shoehorning MA15+ games into the M rating.