ACT will not stand in the Mount Roskill by-election, which is likely to be held before Christmas.
The incumbent MP, Phil Goff, will be resigning from Parliament after being elected mayor of Auckland.
The by-election to replace him looks likely to be held on 3 December.
ACT will not stand in the by-election, with leader David Seymour saying they wanted to give National the best possible chance of winning the seat.
Michael Wood has already been selected as the Labour candidate.
Prime Minister John Key said National MP Parmjeet Parmar, who stood in the 2014 general election, had not been formally selected but she would be a good candidate.
The Green Party said it would not field a candidate because it did not want National to win the seat.
New Zealand First was expected to make a decision on whether to stand in coming days.