In the 2010 House of Representatives election there was a national informality rate of 5.55 per cent. This was the highest informality rate recorded since 1984, and represents a substantial increase from the 3.95 per cent recorded at the 2007 House of Representatives election.Ballots with "incomplete numbering" are, of course, ballots where the voter has clearly indicated at least their first choice but failed to preference all of the other candidates. It's clear who these people were voting for, but due to the regulations put in place by the Coalition government these votes are not counted. Instead of the previous AEC approach - counting any vote where the intention of the voter was clear - the Tories insisted that valid votes must follow exactly the voting instructions provided. Their reason for so insisting is pure politics, nothing but an attempt at voter suppression, based on the assumption that people likely to make errors are either less educated or of a non-English speaking background - i.e. more likely to vote Labor.
...
More than half of all informal ballots in 2010 had incomplete numbering or were totally blank (27.8 per cent with a number '1' only, 2.6 per cent with other forms of incomplete numbering and 28.9 per cent blank).
5.5% voted informally out of a voter turnout of 13,131,667, of which 30.4% failed to number all boxes after numbering at least one. More than a quarter of the informal voters were people who wrote "1" next to their preferred candidate and then stopped. That's about two hundred thousand people who clearly expressed their choice of candidate, but whose votes were ignored.