@JTT,
Yep - it's compulsory voting, but not compulsory voter registration. There is a fine - but any half decent reason will get you out of it.
From the AEC site:
What happens if I do not vote?
Initially the Australian Electoral Commission will write to all apparent non-voters requesting that they either provide a reason for their failure to vote or pay a $20 penalty.
If, within 21 days, the apparent non-voter fails to reply, cannot provide a valid and sufficient reason or declines to pay the penalty, then prosecution proceedings may be instigated. If the matter is dealt with in court and the person is found guilty, he or she may be fined up to $50 plus court costs.