You have to register before they know you exist. Once registered you are required to keep your address updated.
On Election day you turn up at your local polling place and show ID, and the poll worker crosses your name out of a big book and gives you a ballot.
At the end of the day, anyone whose name has not been crossed out gets a $20 fine, unless they can give a reason they didn’t vote.
It leads to comedy pieces like this, from a program called The Chaser. (Language Warning.)
http://www.youtube.com/watch?v=MaQJj1BQUiU
Oh the urge to not pay the fine and wait for them to come...