We have a secret ballot[+], so there is no requirement to actually write anything on the paper. Spoiling your ballot is not illegal (otherwise you couldn't have a secret ballot). But it is mandatory for you to show up, receive a ballot, and put it in the provided ballot boxes.

In Australian elections there are no votes on issues, it's purely deciding your representative in the Senate or House of Representatives (which are voted on separate pieces of paper so you can spoil one and not the other if you wish). Votes on issues aren't really a thing we do outside of referendums (and there hasn't been one of those for a while).

[+] Interestingly the American secret ballot system is called the "Australian ballot".