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This comes after police last week referred the matter to the SFO. It took the office just eight days to decide that an investigation was warranted.
New Zealand First Leader Winston Peters would not comment in depth when stopped by The New Zealand Herald after his caucus meeting this afternoon.
But he did say: "I'm happy for this inquiry to go ahead, that's what we expected."
Act Leader David Seymour said Peters should be stood down by Prime Minister Jacinda Ardern while the investigation is carried out.
"It's impossible to imagine any other Prime Minister tolerating such behaviour from her deputy.
"This is bigger than Winston Peters or Jacinda Ardern it is about the respect owed to the traditions of our country."
Last week, Peters was adamant that the SFO wouldn't find any wrongdoing on behalf of him, or his party.
"I was not involved in any way, shape or form," he told media last week.
Last year questions were raised about how the Foundation handled donations.
The Electoral Commission said at the time it was looking into the matter and on February 10 this year handed the probe over to police.
Peters said he was pleased that the matter had been deferred to police, then to the SFO.
But he said NZ First was going to wait and co-operate with the SFO to provide all the information.
He said he was not subject to any investigation, nor was New Zealand First, "so there is the end of it."
Peters said the SFO would not find any evidence of wrongdoing because no such evidence exists – "we have a legal opinion to back up what we have said".
Late last year, Stuff reported that the NZ First Foundation appeared to have hidden political donations worth almost half a million dollars between April 2017 and March 2019.
Many of these apparent donations to the foundation do not appear on the party's electoral returns.
The issue was referred to the Electoral Commission, then to police, then to the SFO who today decided to investigate.
Recently, RNZ have been reporting specific details about the Foundation's books. The broadcaster has been reporting who some of the donors are, and how much has been donated - much of it just under the $15,000 anonymity threshold .
This is not the first time New Zealand First has been the subject of an SFO investigation.
In 2008, NZ First was investigated by the office because of similar issues with donations to the party.
Back then, Peters - then Minister of Foreign Affairs - stood aside as the SFO investigated issues to do with party donations.
The SFO said at the time it had no basis for fraud charges to be laid, but questions remained about possible breaches of electoral law over the non-disclosure of donations.
"When it was all over," he said, "Winston Peters was utterly exonerated - those are the facts."
He said that would, indeed, again be the outcome of any potential SFO probe today.
Speaking to Magic talk this morning, before the SFO statement was released, Peters said he had never been contacted by the Electoral Commission or the SFO.
"If I was in any way involved, they would have spoken to me, wouldn't they?" he said.
He called the saga a "hymn of hate" and said that he was "going to get above that".
Peters told the programme that the SFO had not been in contact with his office, but he was sure "they would no doubt write to us and will co-operate to the full."