The Ministry of Social Development has cleared its staff of leaking NZ First leader Winston Peters' superannuation details.

The department's stance follows that of Inland Revenue, which has also said its staff did not leak the information.

"Following information regarding Mr Winston Peters' superannuation payments entering the public arena, the ministry launched an investigation to assess whether there was any indication that a ministry employee may have been the source of the information," MSD said in a statement.

"That process is now complete, and we can confirm that all staff that had access to the relevant information had a reasonable business purpose for accessing it, and there is no evidence that this information was passed to a third party.

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The ministry holds a great deal of very personal information about people and their families that New Zealanders trust us to safeguard."

MSD said data searches and staff interviews were conducted as part of its investigation.

Ministerial Services is still looking into the handling of the information by the ministerial offices 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 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, 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".