United Future leader Peter Dunne has hinted he may seek more concessions from National if New Zealand First leader Winston Peters wins the Northland by-election.

Peters confirmed he's standing for the seat, which has been left vacant by Mike Sabin's resignation.

If Peters were to win next month's vote, National would lose its majority in Parliament and would have to rely on United Future or the Maori Party, along with Act, to pass contentious bills.

Mr Dunne signed a confidence and supply agreement with National after the election, but says if the balance of power shifts in Parliament after the by-election he could seek a new deal.

Mr Dunne has previously expressed concern over planned changes to the Resource Management Act and the Government's lack of discussion with its partners on the issue.

But, ACT is not impressed with Peters' interest in the by-election.

It means New Zealand First, Labour, National, and ACT are all in the running.

ACT's candidate Robin Grieve is questioning Mr Peters' motives.

"They haven't stood a candidate in Northland in the last three elections, it clearly hasn't been a priority for them," he said.

Grieve says the area clearly isn't a priority for New Zealand First.

"I think the sudden interest is the fact that because there's a by-election there'll be a spotlight and Winston wants a bit of it."

"That's the only thing I can put it down to."

National will announce its candidate today, while Andrew Little has ruled out any deal with NZ First.