The Green Party has decided to stand aside in the Mt Roskill by-election, giving Labour a better chance of retaining the seat if Phil Goff wins the Auckland mayoralty.
Mr Goff is the hot favourite to become Auckland's next mayor in October, and if he does there will be a by-election in the seat he has held since 1981.
"The Mt Roskill by-election will be closely contested, and we don't want to play any role in National winning the seat," Greens co-leader Metiria Turei said on Tuesday.
"This decision shows the memorandum of understanding between Labour and the Greens is working."
The two parties signed the MOU in May this year, agreeing to work together up to the next general election.
Ms Turei says the Mt Roskill decision doesn't have any bearing on any other seat in next year's election.
Mr Goff is a popular electorate MP and retained Mt Roskill with a majority of more than 8000 votes in the 2014 general election.
But National won the party vote by a majority of more than 2000.
The Greens' candidate won just over 1600 votes, coming third behind Mr Goff and National's Parmjeet Parmar.