New Zealand First leader Winston Peters has accused the Serious Fraud Office of acting with malice, following his latest appearance before the Privileges Committee.
Mr Peters spent about an hour behind closed doors responding to a submission from the SFO, which is investigating donations to New Zealand First.
He has revealed an anonymous letter, apparently from an SFO staff member, written just a week before that investigation was launched.
It criticised New Zealand First MP Ron Mark over a Bill which would abolish the SFO - a move which is opposed by National, and is now on hold while the Office investigates New Zealand First.
Mr Peters said the SFO has exceeded its powers by getting involved in the Privileges Committee process, and its investigation is politically motivated.
He wrote to the committee asking to present more evidence and demanding that the Serious Fraud Office also appear before it.
Meanwhile, the director of the SFO, Grant Liddell, has rejected any suggestions of impropriety or bias in his decision to investigate donations to New Zealand First.
He says he has not seen the anonymous letter so it could not have influenced his decision.
But he says he will be taking immediate action to see if the letter did come from an SFO staff member.
The Attorney General, Michael Cullen, whose responsibilities include the Serious Fraud Office, has declined to express confidence in the SFO's director.
Dr Cullen says he will raise the matter with the director of the SFO on Monday.
Prime Minister Helen Clark on Friday also raised questions about the letter.
"I understand that the letter is identifiably from a member of the SFO staff and of course that's completely unacceptable behaviour for any public servant," she said.
New Zealand First MP Ron Mark said on Friday the letter was an underhand act.
"This is highly unusual, it breaches the procedures, it actually tries to circumvent the political process."
SFO's investigation
Radio New Zealand understands the Serious Fraud Office has made a submission to the committee, which is looking into a $100,000 donation to Mr Peters' legal costs by expatriate businessman Owen Glenn and whether he should have declared it.
The SFO is conducting its own investigation into New Zealand First donations from Wellington businessman Sir Robert Jones and the Vela horseracing family.
Police are also investigating the donations row.
The Privileges Committee met for two hours on Thursday afternoon, behind closed doors.
It is the fourth time Mr Peters has appeared before the committee.
The committee last met on Tuesday, when it heard further evidence from Mr Peters' lawyer, Brian Henry, regarding Mr Glenn's donation.
Mr Glenn last week told the committee that it was Mr Peters who asked him for the donation in 2005, not Mr Henry. On Tuesday, Mr Henry altered key parts of his story but still insisted that Mr Peters knew nothing about the donation until July this year.
