If voting is mandatory in Australia, do they include a "none of the above" choice on the ballot? The commonly accepted way of indicating that sentiment is to spoil the ballot, but giving an explicit choice might be an interesting experiment. Of course if that choice won a majority, I'm not sure how anyone would be better off.
I'm not sure how useful that would be, but it might serve some purpose to get an idea of the extent of the "donkey" vote. At present, for the Federal elections and most State elections, we have a preferential voting system. The ballot paper contains a list of all the candidates for a particular seat, maybe half a dozen or so, and the voter is supposed to place a number in each box next to each candidate's name (from 1 to 6) in order of preference.
When a reluctant voter who may not be interested in politics and doesn't want to think about the issues, places his preference in numerical order from 1 at the top of the ballot paper to 6 at the bottom of the ballot paper (instead of, for example, 314562 which might indicate a thoughtful vote), the vote is described as a "donkey" vote.
However, in situations where there are fewer candidates for a particular seat, the percentage of donkey votes becomes less certain.
The other issue is in the definition of compulsory voting. It's really a misnomer. What is compulsory is attendance
at the voting booth. If a voter wishes to fill out the form incorrectly, or deface it, thus making the vote invalid, he's free to do so. There's no-one looking over his shoulder as he fills in the ballot paper.
I believe such 'informal' votes, as they are known, constitute about 4% to 5% of the total votes on average, but who knows how many donkey votes there are! Impossible to calculate precisely.