Earlier quoted context omitted.
Australia is behind the times in this respect. Voting is a right, not an obligation and non-participation is a form of protest against the system as a whole rather than a forced choice between multiple options none of which you may agree with.
Did you read the parent? You don't have to vote in Australia. You just have to show up and not vote -- or alternatively accept a trivial fine. (I personally think mandatory voting is a crappy system, but less crappy than all the alternatives. Much like capitalism.)
The situation (quoted from the WP page) is precisely this:
"Compulsory for federal and state elections for citizens 18 years of age and above. The requirement is for the person to enroll, attend a polling station and have their name marked off the electoral roll as attending, receive a ballot paper and take it to an individual voting booth, mark it, fold the ballot paper and place it in the ballot box."
So your participation in the election process is mandatory.
I'd wager the vast majority of the people so compelled to show up at the polling stations will actually vote. This should be easily testable. The proof would be that in Australia the total number of votes as a percentage of the population is high compared to other countries that do not have mandatory participation and that the number of blank or defaced ballots slightly higher than it would be in a country without compulsory voting (those would be the people actually using their vote as a protest vote).
edit:
http://www.idea.int/vt/countryview.cfm?CountryCode=AU (Australia)
http://www.idea.int/vt/countryview.cfm?id=216 (Thailand)
http://www.idea.int/vt/countryview.cfm?id=164 (Netherlands)
http://www.idea.int/vt/countryview.cfm?id=231 (USA)
Seem to solidly confirm my suspicion.