You can submit a blank ballot. Since the ballot is secret, compulsory voting merely verifies that you got a ballot and submitted it, not that you filled it out correctly/completely.
63.9% turnout is just business as usual, but - if you ask me - 36.1% of ballots being spoiled would precipitate a sea change in the political landscape.
An alternative would be a national holiday rather than mandatory voting. Either way, I'd like to see voting made easier.