Some journalists involved in a leak of Winston Peters' pension over-payments weren't acting as reporters, but were instead part of a political hit job, the NZ First leader's lawyer says.

During this year's election, the now deputy prime minister admitted, after questions from media, that he had been overpaid his pension for seven years.

He has started legal proceedings against senior National party figures and journalists to find out who leaked his information from the Ministry of Social Development.

At the first hearing into the matter - at the High Court at Auckland on Monday - Peter's lawyer Brian Henry said the case was not just a matter of dirty politics but about a "political set-up".

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"We are talking about an illegal act," he said.

While some journalists had received the leak but not acted on it, others had, and Peters would be looking to challenge their journalistic privilege, arguing they had acted more like political operatives, Henry said.

"When it comes to the journalists, it is our understanding some of the journalists were not 'journalists'."

He said it was not yet clear how Newsroom co-editor Tim Murphy - one of the defendants - had been involved.

"I don't know whether he was acting as a responsible journalist, or just big-mouthing around town, or involved in a political set-up."

In the days before Peters disclosed his superannuation situation, Murphy had tweeted about an upcoming "mother of all scandals".

Murphy's lawyer, Andy Glenie, told the court Peters' request for disclosure of documents and records didn't specify who he ultimately planned to take action against.

"It's not clear which parties are in the gun," he said.

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Justice Anne Hinton has ordered a draft claim be submitted to clarify Peters' plan.

She also ordered an affidavit from Peters, after Henry said his client had been "slightly busy".

"Since the election we haven't been able to get near him," he told the court.

The case will return to court on March 5.

Since Peters announced the legal action last week, media freedom groups have warned it could have a chilling effect on political reporting.

National's Bill English, Paula Bennett, Steven Joyce, Anne Tolley, Ministry of Social Development chief executive Brendan Boyle, English's former chief of staff Wayne Eagleson, National party staffer Clark Hennessy and journalists Murphy and Lloyd Burr have all been named in the case.

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All National members deny involvement in the leak.

- NZN