A great place to start is the Library's Law & Politics eResources. From here, you will find a range of freely available Australian legal sites, as well as licensed resources available offsite to registered readers, and onsite resources available inside the National Library building only.
ComLaw is maintained by the Australian Attorney General's Department and contains the Federal Register of Legislative Instruments.
Australasian Legal Information Institute (Austlii) provides access to Australian primary and secondary legal information. It contains most fulltext legislation and court decisions.
Information on Australian Commonwealth and state legislation held in Australian libraries can be found on GovPubs: the Australian Government Publications Guide (please note that GovePubs is no longer maintained).