Tom, I agree with you that we should not have compulsory voting here, however they don't sling us in gaol.
Mind you, I was surprised at what can happen if you don't pay the fine. During a period after I moved and my mail redirection had run out, I was apparently sent a fine for failing to vote, which must have gone to my old address.
It wasn't until much later, when they apparently caught up with where I was through the electoral roll, that I received a rather startling notice from a collection agency in the mail which said in big black letters "Suspension of drivers licence", without saying WHAT fine I was supposed to have not paid. Of course this arrived late on a Friday afternoon, as all worrying letters seem to, when you can't find out anything about them till Monday morning.
When I eventually did find out what it was all about, I was told it was for failing to vote. All I could manage was "You're kidding! They can suspend your licence for THAT?" Of course by that time the fine had gone up from $25 to $125, and it was too late to protest that I never received the original fine. So I was FORCED to pay it because I need my drivers licence. That made me very angry. Yeah, so much for democracy!