Labour leader Phil Goff has admitted National's Hekia Parata could win the Mana by-election if turnout is low.
His press secretary, Kris Faafoi, is standing for the seat, traditionally seen as a Labour stronghold. But yesterday Mr Goff said a low turnout would "jeopardise Labour's hold on the seat".
Senior National party sources also said yesterday they were expecting a poor rate of participation, which would close the gap with Labour.
Mr Goff said: "On issues, in terms of work put in by the candidate and in terms of party support, Labour should win the by-election.
"But if people don't turn out to vote that would put a victory in jeopardy.
"The way in which the National candidate [Hekia Parata] could win is if there is a very low turnout. She's run before, she's reasonably well known there, but it really depends on turnout."
He admitted union boss Matt McCarten, an independent, could split the vote. "If people decide to vote for Matt, they won't get what they want, they'll get a National member of Parliament."
But he added: "I think a lot of people have figured that out."
Registrar of Electors Toni Weir has urged voters to register. Enrolment closes on Friday.
Can the ACT Party survive?Related story: ACT life support still on