Inland Revenue has concluded an investigation and found its staff members were not the source of a leak of New Zealand First leader Winston Peters' private information.
The department said it does not hold the information about the MP's superannuation that became public, so 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.
Mr 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 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 came 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, Mr 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".
Mrs 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.
Ms Tolley was told by MSD chief executive Brendan Boyle on July 31 and got an update from MSD on August 15. Ms 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 told on June 19 as part of a routine briefing. On July 27 he was advised the matter had been resolved to officials' satisfaction.
Mr 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 Ms 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 Mr Peters paid was $18,000 and the overpayments were detected when Mr Peters' partner Jan Trotman applied for superannuation.
It remains unclear who was at fault for the overpayments. Mr Peters was on the rate for a single person who was sharing accommodation rather than someone living with a partner.
The MP has said he took Ms Trotman with him when he applied for super in 2010, but neither MSD nor Mr Peters will release his initial application form or reveal how much he had to repay.