Reaction from Winston Peters' political opponents to the Serious Fraud Office investigation has been swift and merciless.

National leader John Key said the Prime Minister should stand Mr Peters down immediately.

"She should stop prevaricating. He's the Foreign Minister of New Zealand, he's under investigation on numerous fronts. It is a highly unusual action for the Serious Fraud Office, and if she's going to maintain what is left of the integrity of her Government, she has to stand him down now."

On Tuesday, Mr Key made his own decision, ruling Mr Peters out of any post-election coalition unless there was a credible explanation for the allegations around the donations.

Act leader Rodney Hide - whose complaint to the SFO sparked the investigation - also said Mr Peters could not continue.

"If having a minister mislead her and mislead Parliament isn't enough, having a Minister of Foreign Affairs under investigation by the Serious Fraud Office - well, what sort of look does that give New Zealand? Imagine the briefing notes other countries are getting on the Minister of Foreign Affairs as they come to our country."

Mr Hide said the SFO was the only body with the powers to force Mr Peters to prove there was no wrongdoing.

"Winston's bluff and bluster won't work on the Serious Fraud Office. He won't be able to decide when he answers the questions, or how he answers them."

Mr Hide has also handed evidence relating to the Simunovich fisheries inquiry of 2003 to the Serious Fraud Office, including a tape of an unbroadcast TVNZ interview with former skipper Wayne Crapper.

Maori Party co-leader Pita Sharples said Helen Clark had previously stood down ministers under investigation, including John Tamihere.

"One would think she would be consistent, but once again this is her call."

But he said he had no opinion on it, saying it was "politically loaded".

"We are so close to the election, there's a couple of bills the Government needs to put through and Winston's party is right there in the middle supporting that. It's quite a political drama and I'm quite interested in just watching it play out.

"If the SFO, with Rodney leading the charge, feel they have a case and there is a case to be tested, they obviously do and will go through the necessary process."

Green co-leader Russel Norman said it was obvious the Prime Minister had to consider standing Mr Peters down.

But Mr Peters should first "seriously consider" standing down himself for the duration of the investigation.