From the AEC website:
http://www.aec.gov.au/voting/Compulsory_Voting.htm
Compulsory Voting
Updated: 18 May 2011
History of compulsory voting in Australia
•Compulsory enrolment for federal elections was introduced in 1912
•Compulsory voting for state elections was introduced in Queensland in 1915
•Compulsory voting at federal elections was introduced in 1924.
Arguments used in favour of compulsory voting
•Voting is a civic duty comparable to other duties citizens perform e.g. taxation, compulsory education, jury duty
•Teaches the benefits of political participation
•Parliament reflects more accurately the "will of the electorate"
•Governments must consider the total electorate in policy formulation and management
•Candidates can concentrate their campaigning energies on issues rather than encouraging voters to attend the poll
•The voter isn't actually compelled to vote for anyone because voting is by secret ballot.
Arguments used against compulsory voting:
•It is undemocratic to force people to vote – an infringement of liberty
•The ill informed and those with little interest in politics are forced to the polls
•It may increase the number of "donkey votes"
•It may increase the number of informal votes
•It increases the number of safe, single-member electorates – political parties then concentrate on the more marginal electorates
•Resources must be allocated to determine whether those who failed to vote have "valid and sufficient" reasons.