We have very comprehensive data on that because we report on the level of informal votes.
Somebody can cast a party vote and not mark a candidate, so that's a valid party vote and an informal candidate vote. They can also—although this happens less—mark their candidate vote but not the party vote.
On average, about 0.45% of party votes are informal and just over 1% of candidate votes are informal, so the level of informal voting is very low. The level of candidate informal voting is higher because often people will find a party they want to vote for but don't have a candidate standing in the electorate for the party that they want to vote for. As a result, often you will find that the party vote side of the ballot has been marked but the candidate vote side has not.
At the 2014 election, 31% of New Zealanders voted strategically; in other words, in the party vote they voted for a certain party and in the candidate vote they voted for someone from a different party.