More than 19,500 votes were invalid in the past three Bay of Plenty District Health Board elections.
The district health board uses Single-Transferrable Vote (STV), a method of voting Tauranga City Council is switching to this year.
But the city's electoral officer says invalid votes are largely caused by having a mix of voting methods on a single paper, not STV itself.
In previous elections, Tauranga City Council used First Past the Post, where voters give the candidate of their choice for each seat a tick. Those with the most votes, win.
It's also the method the Bay of Plenty