Union leader Matt McCarten is standing as a candidate in the Mana by-election as an independent.
The by-election will be held on November 20th.
The Labour party are standing former TVNZ reporter Kris Faafoi. Hekia Parata, a list MP, is National's candidate.
''The [Unite] executive signed off on it last night,'' said Mr McCarten, who is battling cancer.
''We've been thinking about how to engage on the issues.
''That Labour party will have to work for their vote now ... and Hekia will get all excited.''
He said Labour were ''going in the right direction'' but the union had decided to back him.
Mr McCarten was originally a Labour party member before becoming president of Alliance. He also did campaign work for the Maori party.
He has been at the forefront of the union's campaign against the 90-day employment probation scheme.
Earlier this year he announced he had a series of operations to treat his cancer.
At the time he said: ''I'm not incapacitated. It's been a nuisance, but it hasn't stopped me doing anything.
My brain is still clear, I can still get around. It's not like I'm in pain or anything, particularly."
The by-election is taking place because Labour MP Winnie Laban left Parliament to take up a position at Victoria University.
It is considered a Labour safe seat.
Mr McCarten will hold a press conference in Porirua this afternoon to detail what he will be campaigning on.
- The Dominion Post
Should the speed limit be raised to 110kmh on some roads?Related story: 110kmh limit moves closer