Inland Revenue and the Ministry of Social Development have launched investigations into whether there was a leak of NZ First leader Winston Peters' superannuation payments.

State Services Commissioner Peter Hughes announced the investigations, saying there would be "zero tolerance" for any leak by a public servant.

Hughes also confirmed he had briefed State Services Minister Paula Bennett on Peter's overpayments on super after discussing whether it should be declared under the "no surprise"' practice with Ministry of Social Development chief executive Brendan Boyle.

Boyle had told Social Development Minister Anne Tolley in late July.

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"There was an expectation that these matters would be held in confidence by ministers."

He said "very limited" details were given and the ministers were not told until after all decisions had been made by the department.

"That ensured there could not have been inappropriate involvement in operational decisions, while allowing ministers to be aware of significant matters in their portfolio."

He defended the decision to go to the ministers, saying the "no surprises" convention existed to ensure ministers were informed of issues that might be controversial or the subject of public debate and said he and Boyle had sought advice from the Solicitor General.

"My advice to Mr Boyle was that MSD should deal with Mr Peters' case in line with the agency's standard policies and procedures in exactly the same way as would happen for any other New Zealander. I am assured that is what happened."

He said it was "unacceptable" for a public servant to leak information.

"Leaking is a political act which would compromise the political neutrality of the public service."

Peters said today the leak was "dirty politics" and he was determined to get to the truth of who was behind it.

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"It's under-hand, it's dirty politics and it's in my view illegal and utterly wrong - and apparently they [members of the Government] knew before I knew."

Inland Revenue confirmed is was investigating whether information about the NZ First leader's superannuation was leaked from the department.

Acting Commissioner of Inland Revenue Cath Atkins confirmed this morning that Inland Revenue was looking into an allegation the superannuation leak came from inside Inland Revenue.

Inland Revenue takes allegations of this kind very seriously, Atkins said.

"And it's important to point out that under the tax secrecy provisions [section 81] of the Tax Administration Act, the Minister of Revenue would not be briefed, and had not been briefed, on this matter as it relates to an individual taxpayer," she said.

The allegation was made and then withdrawn by a journalist yesterday, she said. Even though it had been withdrawn, IRD would still investigate.

"To preserve public trust and confidence, we need to be satisfied that there is no evidence of wrongdoing. That work is ongoing."

Inland Revenue has very clear code of conduct requirements that all staff were aware of, Ms Atkins said, and very strict tax secrecy obligations in respect of individual taxpayer affairs.

"Until we're able to discover more about the allegation, Inland Revenue will not be commenting any further," she said.