Prime Minister Helen Clark says she will not step into a row between New Zealand First leader Winston Peters and The New Zealand Herald over its claim that expatriate businessman Owen Glenn made a donation or loan to the party.
Mr Peters has dismissed the claim as fiction and is demanding an apology.
He says his party can find no evidence of receiving money from the Mr Glenn.
National Party leader John Key says Miss Clark must ensure Mr Peters has been truthful about the claim.
Mr Key says the issue is not just a matter for New Zealand First because Mr Peters is also minister of foreign affairs. Mr Glenn has expressed his interest in becoming the honorary counsel in Monaco, where he is based.
Mr Key says Miss Clark must demand the same accountability from Mr Peters that she expects with other ministers.
But Miss Clark says it is an issue for New Zealand First and Mr Peters.
Mr Glenn's public relations adviser, Steve Fisher, says he is unable to confirm whether Mr Glenn made a donation or loan.
Mr Glenn was the Labour Party's biggest donor at the 2005 election.
The New Zealand Herald story says Mr Glenn admitted in an email to making a donation.
The email was written to Mr Fisher who said on Monday that when the matter first arose in February, he advised Mr Glenn to refer all questions to New Zealand First.
Mr Fisher says he no longer has the email, cannot confirm its wording and does not know how it was obtained by the newspaper. He says he cannot remember if Mr Glenn told him he had given money to the party.
Mr Peters says the party has no records of any loans or donations from Mr Glenn, who is based in Monaco.
Mr Peters says the newspaper's editor and the journalist who wrote the story are welcome to go through the party's accounts.
Mr Peters was questioned about Mr Glenn in February after then-party president Dail Jones said New Zealand First had received a five-figure anonymous donation.
Mr Peters said at the time that the money was a consolidation of party funds and Mr Jones was mistaken.
But the paper says it stands by its story and the journalist who wrote it.
Editor Tim Murphy says the story was based on explicit emails and Mr Peters' response has left open to interpretation possibilities of funding assistance for New Zealand First and its interests.