Prime Minister Jacinda Ardern is confident Deputy Prime Minister Winston Peters will not be part of any Cabinet decision-making around his legal case against public servants while he's acting as prime minister.

Peters has begun new legal action seeking $450,000 for alleged breach of privacy in relation to the leaking of details of his superannuation overpayment.

Ardern told RNZ today that she found about her deputy's latest action yesterday.

Peters is going after the Ministry of Social Development chief executive Brendan Boyle, State Services Commissioner Peter Hughes, the Attorney-General on behalf of the Ministry of Social Development and former ministers in the previous National government Paula Bennett and Anne Tolley.

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Ardern said the legal action, which comes on the eve of her six weeks' maternity leave, was a private matter for Peters.

"It is a matter for him as an individual that he is pursuing this. In the last eight months in which he has held his political role it has not affected his ability to do his job, nor do I expect it will in the future."

All targets of Peters' action are entitled to apply to the Cabinet to have their legal fees paid - the Cabinet which Peters will be leading.

Ardern was not concerned by any conflict of interest arising for Peters.

"I would expect that in any case where there could be any potential conflict with this case, that the minister would recuse himself from any decision-making. That is what the Cabinet manual sets out and that's what will happen."

A spokesman for Peters said yesterday: "The minister's view is this a private legal matter which is now lodged before the High Court. He won't discuss matters which are sub judice."

Some previous targets for Peters are not named in this action, namely former National Party leader Sir Bill English, former minister Steven Joyce, former chief of staff Wayne Eagleson and National Party staffer Clark Hennessy.

Peters has targeted journalists Tim Murphy, editor of Newsroom, and Lloyd Burr, a former political reporter at Newshub, in a previous action for discovery of documents but is not pursuing them.

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It is understood that papers were lodged in the High Court at Auckland on Monday.

About four weeks before last year's election, Peters revealed he had been overpaid the old-age pension for seven years, but that he had repaid the undisclosed amount as soon as he had found out.

He proactively revealed the overpayment in a press statement because media outlets had been asking questions about it.

Peters first accused IRD of having leaked the information but then turned his sights on Boyle, then political rivals.

Boyle sought the advice of his employer, State Services Commissioner Peter Hughes, as to whether such information should be passed on to minister under the no-surprises policy.
Hughes in turn had consulted Solicitor-General Una Jagose before Tolley and Bennett were told.

Tolley said she had also informed Eagleson, who was chief of the staff to the then Prime Minister Sir Bill English.

In Official Information Act documents obtained by Newsroom into the internal MSD investigation, it transpired that 41 MSD staff had been involved in Peters' case and interviews had been conducted with 12 staff who had full access to information on the case.

National has consistently denied having leaked details.