No, they don't get an automatic ban.
We have our own classification group called the OFLC (Office of Film and Literature Classification) which rates various forms of media which is released down here in Australia.
The rating system for movies down here is:
G, PG, M15+, MA15+ and R18+
The rating system for videogames down here is:
G, PG, M15+ and MA15+
The great majority of the most violent videogames can fit into the M15+ and MA15+ ratings pretty easily. Probably about 1 game every few years will be made that would really justify the use of the R18+ rating... however, the real issue isn't that we don't have an R18+ but that the lack of an R18+ rating is being used as an excuse to censor our videogames. Of course, with that said... there are certain games that have been released complete and uncut with an MA15+ rating that could probably do with an R18+ rating... but we don't have one!
It's really funny when you get down and look at what kind of reasons certain games are banned and edited. For instance, one game can be banned because the main character doesn't show remorse for his violent actions. However, a game like Resident Evil 4, which has some pretty violent things in it and have been censored in some other countries, is completely fine down here and we got the complete and uncut version of that game.
Fallout 3 was going to be banned because of its use of morphine... and that's it! THAT was the reason. The game has since been edited worldwide with morphine changed to some made up drug name... and suddenly that makes it fine to release down here now.
Silent Hill Homecoming was recently banned down here, though I don't know what for... I wonder what will happen when No More Heroes 2 comes out down here...