Labour Party leader Phil Goff says Hone Harawira's decision to force a by-election in his Te Tai Tokerau electorate is a stupid and unnecessary waste of taxpayer money.
Mr Harawira announced at the launch of his left-wing pro-Maori Mana Party in Auckland on Saturday that he intends to resign as an independent MP and stand as a candidate for the new party.
Mr Goff says that's a publicity stunt by a breakaway MP who's trying to establish himself.
He says it's a waste of half a million dollars of public money, and by the time a new MP is sworn in there'll be only a short while left before the House adjourns for the general election.
Hone Harawira says every MP deserves the right to get a mandate from the people, and that's what he's doing in his Northland electorate.
Former Green MPs Sue Bradford and Nandor Tanczos appeared at the party's launch. Green co-leader Metiria Turei says she does not believe the Mana Party is an issue for her party.
The group Socialist Aotearoa also gave its support, as did Matt McCarten and John Minto from the Unite Union.
The Council of Trade Unions says it's too early to know if the Mana Party could eat into votes which would otherwise go to the Greens or Labour.
The Council says it does not endorse any political party.
But its president, Helen Kelly, says the Greens and Labour have a proven track record of standing up for workers' rights, and Mana is untried.