New Zealand First won't be standing a candidate in the Northcote by-election, in protest against "needless cost".
The by-election on June 9 was triggered when National's electorate MP Jonathan Coleman announced his resignation from Parliament.
"Standing a candidate endorses... Jonathan Coleman walking out on his electorate and needlessly costing the taxpayers a million dollars," said NZ First Leader Winston Peters.
"A National MP's thwarted ambition provides the worst possible reason to abandon an electorate, and Northcote voters will no doubt react in the by-election to the flimsy reason for unnecessarily returning to the polls a mere six months since Northcote voters made their last choice," Mr Peters said.
Fellow support party, the Greens, is standing in the by-election, though a candidate has not yet been selected.
National has selected its candidate: Māori fullbright scholar and economist Dan Bidois.
Shanan Halbert, who contested the seat for Labour in the 2017 election, will be standing for Labour again.
Mr Halbert ran against Dr Coleman in 2017, losing by 6200 votes.
Neither of the two major candidates are well-known names, meaning the fight for the seat is likely to reflect party lines, rather than personality-based votes.
The electorate of Northcote is located on Auckland's North Shore and is relatively affluent, with a third of families earning more than $100,000 a year.
The contest for the seat will be a test of the new Government's popularity.
If Labour decreases National's majority or (in an unlikely scenario) outright wins the seat, it gets another member in Parliament and solidifies its mandate.
If National wins, it can double-down on claims of its own popularity.