United Future leader Peter Dunne says he would look at revisiting his post-election concessions from the National Party if New Zealand First leader Winston Peters wins the Northland byelection.

Mr Peters joined the contest for former National MP Mike Sabin's electorate yesterday, and Mr Dunne said if National lost the seat it could effectively give the balance of power to United Future or the Maori Party.

In announcing his intention to run in Northland, Mr Peters made an obvious pitch for National votes, saying a New Zealand First win "would not change the Government".

National would maintain a majority on confidence and supply with its coalition partners Act, United Future and the Maori Party.

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However, it would no longer be able to pass legislation with the help of Act alone, and would require one more vote from either of the other two parties.

Mr Dunne pointed to Resource Management reforms as one area which he could have greater influence over if National lost a seat. He said the reforms, which he has expressed concerns about, would "take on quite a different hue" if National relied on his vote to pass.

Mr Dunne noted that his party signed a confidence and supply agreement when National had an outright majority. National has since lost a seat in the final election count and could lose another in Northland.

Asked whether he would seek greater concessions from National if it lost another seat, he said: "That's something I'd want to consider. I don't have an immediate answer at this point."

The Northland seat was vacated when Mr Sabin stood down last month for "personal reasons".

He won with a 9000-vote majority in the September election.

Mr Peters, who was born in the electorate, said it had been neglected by the Government.

However, his party has not run a candidate there in the past three elections. It won 12.7 per cent of the party vote in 2014.

National's campaign manager Steven Joyce said voters would question whether Mr Peters was the "fresh, new energetic face" for Northland.

"Forty years in politics ... I think people will look at it as a Winston's greatest hits tour," he said yesterday.

Mr Joyce conceded that Mr Peters had a following in the electorate and said National would "have to take everybody seriously".

Mr Peters' popularity in Northland could hurt Labour, which has chosen lawyer Willow-Jean Prime to contest the seat. Labour leader Andrew Little said yesterday he would not pull his candidate from the race to give Mr Peters a better chance.

If Mr Peters did win the byelection, it would enable New Zealand First's 12th-ranked candidate, Invercargill-based candidate Ria Bond, to enter Parliament.

Northland candidates

Act Party

— Robin Grieve

Labour

— Willow-Jean Prime

National

— selected today (nominees: Mita Harris, Matt King, Grant McCallum, Mark Osborne, Karen Rolleston)

New Zealand First

— Winston Peters

Greens, Maori Party, United Future

— no candidate

Nominations close on Tuesday, byelection held March 28.