Tipster Mick managed to get his hands on the Office of Film & Literature Classification's report for Grand Theft Auto IV. It's not very large at three pages, but it's more than enough for Kotaku to find out what we want to know.
Firstly, the violence. The OFLC doesn't see it as anything out of the ordinary for the MA15+ rating, describing it as "hand to hand combat (basic punching and kicking) [that] more regularly involves use of various weapons". It does make particular mention of what you can do to corpses: "Blood pooling occurs under bodies that are shot at after death however no post mortem damage (such as decapitation or dismemberment) is possible".