It's not compulsory to register to vote.

edit: apologies, that's completely incorrect.

While it is compulsory, you are incredibly unlikely to be fined for not registering. The AEC typically sends a reminder every few years if they have information on your residential address from another government agency.

Once you are enrolled though, if you fail to vote at an election you will be asked to explain your absence and, without a satisfactory excuse, fined.

It looks like the AEC now has the power to automatically enrol you too.