Two people are facing charges after a Serious Fraud Office investigation into the NZ First Foundation.

The pair have been charged with obtaining by deception. The charges were filed on September 23.

"The defendants have interim name suppression and so cannot be named or identified at this time," the SFO said in a statement.

"We note, however, that neither defendant is a minister, sitting MP, or candidate in the upcoming election (or a member of their staff), or a current member of the New Zealand First party."

NZ First leader Winston Peters says all MPs and ministers, candidates and party employees were cleared of wrongdoing: "No party member has been implicated or charged by the SFO," he told reporters.

The SFO charges were now a matter of sub judice and Peters said he couldn't answer whether the two people facing charges were ever party members.

He wouldn't say if he knew the two people who have been charged.

Peters was highly critical of the SFO and said NZ First would now seek a High Court declaration that the SFO had abused its statutory powers.

He said the timing of the decision to lay charges against two defendants of the NZ First Foundation was "an appalling intrusion" and a "James Comey level error of judgment".

"The SFO cannot justify the timing of its decision."

It was one day before overseas voting started and a few days before advanced voting started, he said.

The distinction that the party was "entirely separate" from the foundation would be lost on some, he said.

The SFO had acted unreasonably and without justification in the way in which the investigation was conducted, and NZ First lawyers would seek a High Court declaration that the SFO had abused its statutory powers, Peters said.

It was "an intervention in the election which has no relationship to the mandate the SFO has", he said.

He compared it with the National Party SFO investigation, the SFO probe into Labour mayoral candidates in Auckland and in Christchurch, and into donations to the Labour Party in 2017.

"How is that fair?" Peters said.

He also had a swipe at the media smearing himself and his party.

"We've been through months and months and months of all sorts of people being grabbed and interviewed ... and there was absolutely nothing."

Peters said he was not interviewed by SFO investigators but was sent a series of questions.

"They said in April they were going to give you a decision before the election, which was on September 19."

He questioned why that timeline was given while the other SFO probes into Labour and Labour mayoral campaigns were not on the same timeline.

"This is my second time to be exonerated and smacked before an election."

Asked if the SFO should have sat on its findings until after the election, Peters said there was no precedent to have the SFO announce its findings before the election.

The impact of this decision on the election depended on how the media would have its "months and months of deceit, lies and malice", Peters said.

Whether the timing amounted to political interference was a question for another day, he said.

"We always knew we were innocent, but we put up with it month after month after month."

He said he was told of the timing of the SFO decision on September 23, and had to fight to be told what the agency's decision was.

NZ First at 1 per cent in latest poll

Peters has always said he believed the foundation's work was above board and at arm's length from the party.

Today's SFO announcement comes at a time when NZ First has been struggling in polls, registering only 1 per cent in yesterday's 1 News-Colmar Brunton poll.

Peters has always been dismissive of polls, and his party has generally done better on election day than in polling leading up to it.

On his plan to pursue the SFO in the High Court, he said: "This is a very expensive business. But in the end, honour, integrity and a fair go is worth defending."

Foundation formed in 2017

The NZ First Foundation was approved by the party board in March 2017.

The foundation's activities came under scrutiny over whether it has loaned or provided money to the party for purposes that benefit the party and its MPs, and if so, whether these have been properly declared.

The party's returns show that the foundation was listed as having made a loan of $73,000 to NZ First for 2017, $76,622 for 2018 and $44,923 for 2019.

RNZ reported that the foundation collected donations of more than $500,000 from April 2017 to March 2019.

During that period, the foundation reportedly spent more than $425,000 on campaign advertising expenses, political consultants' fees, renting and setting up a campaign HQ in Wellington, and running the party's website.

Apparent discrepancies between the foundation's accounts and the party's donation returns have raised questions around whether record-keeping was properly done and whether donation disclosure laws were properly followed.

These articles followed the resignation of party president Lester Gray last year after he refused to sign the party's 2019 financial documents.

"This type of operation does not align with my moral and business practice values, and I am therefore not able to support the party any longer," Gray told Stuff at the time.

In February, the Electoral Commission said it believed the foundation "has received donations which should have been treated as party donations for the New Zealand First Party".

The matter was referred to police, and then the SFO, which said that its investigation would conclude in time to be in front of voters before the original election date of September 19.