Australia has mandatory voter turnout, but you do not need to submit a vote. You just need to show up on polling day.
So just to clear up a technical misconception here - the wording in the Electoral Act is quite clear. All enrolled electors are legally required to vote. It's only a consequence of the secret ballot that makes this provision unenforceable, so someone can turn up and get their name marked off while not submitting a vote without facing any consequences, but it is technically an illegal act.
If the AEC were to come up with some way to determine that you didn't vote without betraying that secret ballot, they would be within their rights to issue a you a fine.
So just to clear up a technical misconception here - the wording in the Electoral Act is quite clear. All enrolled electors are legally required to vote. It's only a consequence of the secret ballot that makes this provision unenforceable, so someone can turn up and get their name marked off while not submitting a vote without facing any consequences, but it is technically an illegal act.
If the AEC were to come up with some way to determine that you didn't vote without betraying that secret ballot, they would be within their rights to issue a you a fine.