Winston Peters is offering to give $100,000 to charity if Prime Minister John Key can find anyone who has seen him fishing from his boat since the Northland by-election.
The NZ First leader issued the challenge after a spat in Parliament on Wednesday.
They were arguing about Northland's economic development and Mr Key said Cabinet minister Steven Joyce went there last week to announce a 58-point plan.
"I dare say minister Joyce knows more about Northland than the member does, and having been there once this year he's probably been there more than the member has," Mr Key said.
Mr Peters retorted: "Is it not a fact that I've spent more time in my electorate in nine months than he has for the whole time he has been the MP for his electorate?"
Mr Key: "Not if you count time in your electorate being on a boat fishing."
Mr Peters has issued a statement accusing Mr Key of "a new element of misinformation" in Parliament.
"I challenge the Prime Minister and I will pay to his favourite charity, not himself, $100,000 if he can find one witness that has seen me out on my fishing boat, fishing, since the Northland by-election," he said.
"He won't, of course, which is why he should apologise, pay $100,000 to a Northland charity of my choosing and stop wasting Parliament's question time with such blatant untruths."