Former Police Association Greg O'Connor has won the Labour nomination in Ohariu.
O'Connor will take on United Future leader Peter Dunne, who has represented the area for 32 years. He said he expected a "formidable contest" in the electorate.
In one of the closest races in the 2014 general election, Dunne won by just 710 votes over Labour's Virginia Andersen. Andersen is now running in Hutt South against National's Chris Bishop.
Prime Minister Bill English has said National is likely to do a "deal" in Ohariu by telling National supporters to vote for Dunne while giving National their party vote.
Labour leader Andrew Little, on the other hand, has ruled out doing any deals with the Greens under the two parties' Memorandum of Understanding. There had been speculation Labour or Greens could pull their candidate in Ohariu to give the other party a clear run against Dunne in a marginal seat.
The Green Party is aiming to run a candidate in all 71 electorate for the first time, but is yet to announce any details about its Ohariu nomination.