Well for starters there is an MA+15 rating as the highest rating for video games in australia, which is from the out dated opinion that all video games are made for under 15 year olds. The stupid thing is that games that are rated +18 in other countries are allowed to come into australia as a MA+15 with little or no changes. And the games that get knocked back can at times be bafflng as other games with similar content are allowed. Eg, dead space is allowed uncensored but left 4 dead 2 had to be censored for an australian release because "infected humans" aren't really zombies.
The ratings problem lies with the Australian attorney generals, where it has to be a unanimous decision to implement an R+18 rating in video games. This has been held back by (now former) SA attorney general michael atkinson, who believed that having an r18 rating would encourage developers to put in gratuitous sex, violence, drug use etc in games and make it easier for under 18s to get their hands on the games. It's now the current WA attorney general (don't know his/her name) who caved into a bunch of "games are evil" group and said no to the R+18 rating.
This kind of thinking is completely backwards, by having an R+18 rating it protects minors from accessing the game due to the govt laws that would prohibit the sale of an R+18 games to under 18s, so the only way they could get the game would be if a parent or older sibling bought it for them. By not having it they're allowing easier access to "inappropriate" content to minors as parents see an MA+15 rating and don't think much of it and buy it for their 13 year old, who then annoys the crap out of everyone online with their trash talking.
However this decision goes is actually good for the R+18 rating argument. If it's knocked back then it's more reason for adult gamers to write to their attorney general and constructively criticise (not abuse!! Seriously if you haven't been watching extra credits already start doing it and understand that writing in saying you're going to skullf**k someone just gives credit to the notion that gamers are all 13 year olds who live in a basement and lust over DOA style girls and whos only response to a problem is violence that has been programed into them from playing games!) with how the decision is impeding on the rights of adults to enjoy a game that is aimed at adults and not children. If it gets allowed with no changes then you can argue that the content is not suited for minors but isn't bad enough to be outright refused classification so the answer is an R+18 rating. The ironic thing is that it was the original mortal combat game that actually created the ratings system for video games in Australia. Remeber it's not the australian govt, its the attorney generals!