The political future of New Zealand Foreign Minister Winston Peters appeared shaky Wednesday after prolonged and heated questions in parliament about a cash donation he received from a billionaire businessman.

Prime Minister Helen Clark rejected demands to sack him, but opposition leader John Key, whose conservative National Party is tipped to win a landslide victory in a general election later this year, ruled Peters out as a potential coalition partner.

Peters, who leads the small nationalist New Zealand First party in the House of Representatives, is a veteran maverick politician who has served in governments headed by both Clark's Labour Party and Key's Nationals.

Clark faced persistent questioning in parliament after the release of a letter from billionaire Owen Glenn saying that Peters had asked him for money toward a legal bill in 2005 and later thanked him personally for his 100,000 New Zealand dollars (about 70,000 US dollars).

Peters previously insisted that he did not ask Glenn for money and knew nothing about the donation until his attorney, Brian Henry - who never disclosed the names of donors he solicited - revealed it to him last month.

Parliament's privileges committee is considering the issue after a complaint by opposition lawmaker Rodney Hide that Peters had broken parliamentary rules by failing to declare the donation.

Glenn's letter to the privileges committee was released Wednesday, and Peters immediately issued a letter of rebuttal, saying the billionaire's version of events "does not coincide with my recollections."

Clark told questioners there was "clearly a conflict of evidence on the issue" and she would not comment further until the committee, which is to meet again on September 4, makes its report to parliament.

But Key called a press conference to announce, "On the basis of those letters, I'm stating today Mr Peters would be unacceptable as a minister in a government lead by me unless Mr Peters can provide a credible explanation for the Owen Glenn saga."

He dubbed the chances of Peters providing clear and direct evidence that Glenn had got it wrong as "extremely low" and said Clark should stand him down as foreign minister.

Political commentators described Key's move as a "bold political risk," given the history of Peters, 63, and the ability of his party to hold the balance of power between the two major parties at elections.

Peters has earned a reputation as a survivor in New Zealand politics, having been deputy prime minister and treasurer under a National Party administration before supporting Labour's minority government in 2005 in return for the foreign affairs portfolio.

Glenn is an expatriate New Zealander who lives in Monaco, and reportedly lobbied to be appointed New Zealand's honorary consul to that country.

Clark's Labour Party has acknowledged that Glenn donated 500,000 New Zealand dollars to it before the last election in 2005, and he was made an officer of the New Zealand Order of Merit early this year.