To clarify, donkey votes are perfectly valid. This is where the voter simply fills the boxes the same as they are displayed (first presented candidate gets number 1, second gets 2, etc.). A donkey vote is a formal vote, just one typically driven by the same factors that would drive an intentionally informal one. An example of an informal vote that I encountered during scrutineering a local council election is when one voter drew their own box, labelled it "Tony Abbott", and voted for that.