You get a fine. There are ways to vote in advance if you won't be around on the day. I don't actually know what the rate of people who don't vote is or how seriously the authorities take it. Really, though, it sounds like a self-policing system. People who are lazy and don't follow the law that says they must vote I don't want voting anyway. It's a difficult question. Mandatory voting helps prevent voter fraud and I see the value in elections being based on the votes of everyone and not just the people who bother. It also removes the massive expense that America pays trying to convince people to vote through rallies and speeches. On the other hand, I also see the value in not forcing people who don't care about politics to vote because I don't want them contributing.