Evidence, please.
The empirical evidence from the current regime is that where a game is refused classification, the publisher will almost always make the necessary alterations (toning down certain amounts of gore etc) in order to achieve an MA15+ rating. The current system has thus been reasonably effective -- ensuring that games are made suitable for a 15+ audience, and given that anyone in the 15-18 category is unlikely to be prevented from accessing a title simply by its having a higher rating that is a defensible approach (by which I mean "there is an argument for it" not "it is the correct approach").
To respond to your specific comments -
Children in Australia are very easily able to afford to purchase computer games -- at current prices, a game is likely to be around one to two months' pocket money (not counting additional money from a part-time job, which many 15-18 year olds have).
Regarding BitTorrent, the speed with which a title can be downloaded (ie, the number of active downloaders) isn't actually relevant to availability. There's no part of classification law that says "it's better if you have to leave the download going overnight". The speed of the download isn't difficulty-to-obtain, it's just latency-to-obtain, and I doubt anyone would consider a few extra hours of waiting significant.
In reality, the vast majority of items made illegally available to minors are purchased from shops in defiance of 18+ ratings: cigarettes and alcohol. The number of 16 year-olds who can get a PS3 to play an illegally downloaded game, while large, is much fewer than the number who can get cigarettes illegally from the local store. From an evidence-based perspective, if you want to prevent illegal access by minors, it really is physical availability from shops that should be targeted.
Unfortunately you're spot on: The current system has stopped the most people I know obtaining the types of games that jerk Michael Atkinson doesn't approve of. In terms of his objectives, his policy is completely effective.
The problem is that it's also stopped me playing many games (Left 4 Dead 2 comes to mind -- There's no way I'm spending my money on a second rate version). Why should I, as an adult, be told what I can and cannot do with my free time?
Basically we have to decide:
a) Whether we believe the rating system works & if it doesn't, why we have it at all?
b) What makes games different than any other type of media, such as movies.
c) Whether we accept having our rights restricted for the sake of someone else's kids
(notwithstanding the fact that it's hardly proven that these games create social problems).
I hate to come off as a jerk but I couldn't give a rat's ass about nannying someone else's child. If parents don't want their children to have access to a game I'm playing then they can watch their own children. Furthermore, even if games are detrimental to children, the damage done by games is substantially less than by alcohol or cigarettes so comparisons between the two are mostly nonsense.