Inland Revenue has concluded an investigation and found its staff members were not the source of a leak of Winston Peters' private information.
In a statement, the department said its investigation concluded it does not hold the information about Peters' Superannuation that became public, and therefore could not be the source.
"New Zealanders trust Inland Revenue with their personal financial information. It is essential that we can assure New Zealanders their personal information is respected and protected at all times."
Ministerial Services is now looking into the handling of the information by the ministerial offices involved, and the Ministry of Social Development is also investigating if its staff were involved.
Peters has called in the lawyers and pointed his finger at the National Party as the possible "leak" to the media of the news he had to repay overpayments for his superannuation since 2010, saying it was an attempt to destroy NZ First.
That followed revelations that two ministers - Paula Bennett and Anne Tolley - were briefed on the overpayments by government department heads under a "no surprises" policy.
National leader Bill English has said he did not believe the leak had come from National and was assured by Bennett and Tolley they had not passed on the information.
State Services Commissioner Peter Hughes has defended the decision to inform the ministers, saying it was carefully considered and the Solicitor General had been consulted.
However, English said the government departments should not have told ministers, given the personal nature of the information. He said the ministers had handled it "with integrity".
Bennett has said she knew the information would be "explosive" so decided not to share it even with her staff. She denied National had leaked it, saying there would be little benefit in it for National and it was not the kind of behaviour she would engage in.
Tolley was first told by MSD chief executive Brendan Boyle on July 31 and got an update from MSD on August 15. Tolley also told the PM's chief of staff, Wayne Eagleson, who has denied telling anyone else in the office, including English.
The Ministry of Social Development said Brendan Boyle was first advised on June 19 as part of a routine briefing. On July 27 he was advised the matter had been resolved to officials' satisfaction.
Boyle asked the State Services Commission if it should be disclosed to the Minister under the "no surprises" policy and was told to do so.
That happened four days later on July 31 at a one-on-one meeting with Tolley. It was followed up with a written note on August 15.
Newshub had reported they received an anonymous tip-off by phone three days later on August 18. Newsroom has also reported details of the repayments, claiming the total Peters paid was $18,000 and the overpayments were detected when Peters' partner Jan Trotman applied for superannuation.
It remains unclear who was at fault for the overpayments - Peters was on the rate for a single person who was sharing accommodation rather than someone living with a partner.
He has said he took his partner Jan Trotman with him when he applied for super in 2010 but neither MSD nor Peters will release his initial application form or reveal how much he had to repay.
June 19: Ministry of Social Development officials tell chief executive Brendan Boyle of an issue with Winston Peters superannuation payments.
Mid-July: Winston Peters says he is contacted by MSD and arranges to repay overpayments.
July 31: Boyle tells Anne Tolley at a one-on-one meeting. He has consulted State Services Commission head Peter Hughes about "no surprises" policy.
August 1: Hughes tells State Services Minister Paula Bennett. Says "very limited details" given.
August 18: Newshub get an anonymous tip about the payments, according to the broadcaster.
August 27, 7.05 pm: Peters issues statement on the issue after questions from Newshub.