I wonder if there could be a Negative Vote option?
Two party votes - one for and one against?
Or would that mean National/Labour cancel each other out except for their electorate seats and Green/NZF/Maori/Conservatives/etc split the 60 party seats between themselves? That'd make for some ... interesting ... post-election negotiations.
Agreed, the AUS/NZ difference in informal votes is probably mostly 'protest' votes. According to Australian Electoral Commission report more than half the informal votes in 2010 were completely blank.
Would a 'none of the above' make a difference? It might lower the the number of votes for *anyone* even more. How many people pick the 'least worst' option? Don't know, but it is one of the arguments used for voting at all - i.e. even if there's no-one you particularly want to vote *for*, there are some you object to less than others.
85% was my estimate of the percentage of valid votes cast using the Aust example. However, I miscalculated. Should be 86.4%. Compared against the historic NZ turnout figures I linked to, which might have a 1-2% informal component, thats not too bad. Not brilliant though, and not evidence of compulsory voting being a cure-all.
I went looking for some numbers.
In Aussie the federal fine is $20, but each state also has their own fines, ranging from $55 to $70.
Australia has about the same percentage of unenrolled as NZ. Turnout is for Federal elections is about 93%. Spoiled or invalid votes is about 6% (versus 0.89% in NZ 2011), and has been rising steadily since the 1980s.
On that basis, about 400,000 more valid votes would have been cast in the NZ 2011 election, bringing it up to about 85%. That sounds good, but really only brings it up to the lower end of most elections between 1914 & 1999. Turnout results
On the other hand, there's the cost of enforcement, which would be a few million dollars. The handiest numbers I found were for the ACT in 2012, which has about 256,000 registered voters. 19,000 'please explain' letters were sent to all non-voters. Two reminders were sent to non-responders. In the end 4,500 people paid a fine. Further, 1,000 court cases were started but 630 of them withdrawn (presumably because fine paid or valid reason given). Multiply those numbers by 12 to scale up to NZ.