A quick google found a report on InsideStory on the 2010 general election which revealed that almost 3,252,000 eligible Australians didn’t cast a valid vote.
…an estimated 21 per cent of eligible adults, or over one-in-five, didn’t fill in a ballot paper that could be entered into the count.
This includes people who aren’t registered and who turn up to the polling station and register an “informal vote”. In English, they spoil their ballot papers. It also includes registered voters who boycotted the election.
If we add up those who were eligible to vote but not on the roll, those who were on the roll but did not go to a polling place, the accidental and deliberate informals and the provisional non-admissions, over 3.2 million eligible Australians didn’t cast a vote – extraordinary for a country that has compulsory enrolment and compulsory voting.