There's a wealth of information on this topic online.
There's various provisions under both state specific and Commonwealth legislation such as the Crimes Act and Criminal Code that deal with offences such as stalking, threats to kill etc. These provisions can serve as a protection against online bullying or behavior that may often come to mind when thinking about what specific acts may constitute online bullying.
In terms of specific legislation, you have the Enhancing Online Safety for Children Act 2015 which grants a wide range of powers to investigate complaints related to serious cyber bullying material targeting children. There's also provisions that allow for the removal of material from social media services.
As mentioned above, there's a wealth of information online about this stuff.