Before they introduced compulsory voting in Australia voter turn out had fallen below 60%. So with compulsory voting the "true vote" if you like is total votes minus informal votes minus donkey votes. Voter turn out for Australian elections these days is greater than 94%. If 5% of those votes are informal or donkey votes then that's still 89% of voters completing "true votes". A 30% non-vote has a far greater influence over an election than a 2% donkey vote.

The Australian Electoral Commission has a page about it: http://www.aec.gov.au/About_AEC/Publications/voting/index.ht...