New Zealand First leader Winston Peters could face legal action if he wins the Northland by-election later this month.
Act leader David Seymour says he's looking at challenging the result in court if Mr Peters wins.
"We've got a situation where there is no legal precedent for a seat to change hands from being a list seat to an electorate seat," says Mr Seymour.
If Mr Peters takes Northland, National will only have 59 seats in Parliament. Even with Act's one vote, National wouldn't have the numbers to pass legislation.
NZ First would gain an extra MP if Mr Peters resigns from the list to become Northland's electorate MP.
"If he really thought he was going to do something for the people of Northland, he would resign now then contest but ultimately it's the court's final say," says Mr Seymour.
Mr Seymour's yet to decide whether he'll take Mr Peters to court.
Mr Peters told ONE News he found Mr Seymour's comments laughable. The Electoral Commission says it has no concerns about the implications if Mr Peters wins.
Meanwhile, National has responded to accusations it's trying to buy the by-election with $69 million dollars in bridge upgrades.
Transport Minister Simon Bridges says it would have been announced if Mike Saben was still Northland's MP.
"We would still have seen these bridges. I think the timing of it might have been different, so when we got to that, I don't know," says Mr Bridges.
The seat became available following the resignation of Mr Sabin. The by-election is on March 28.