More than two years after it was publicly revealed he had been overpaid superannuation, lawyers for Deputy Prime Minister Winston Peters say they're set to explain how the error happened.

Peters will tomorrow trade Wellington for Auckland as he takes the stand to give evidence in a privacy lawsuit he is bringing against former National Government ministers Anne Tolley and Paula Bennett, the Ministry of Social Development, the chief of the public sector watchdog, and a top civil servant.

Since 2017, the New Zealand First leader has argued he was the victim of dirty politics when media were told he had paid back seven years of superannuation overpayments.

National has consistently denied being behind the leaks, as has the public service.

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Over the next three weeks, politicians, officials and journalists will front the High Court to figure out whether Peters was wronged and by whom.

But while the core of the case will largely focus on how the information got to reporters, a remaining question from the saga has been how the overpayments began in the first place.

On August 27, 2017 – less than a month out from a general election – Peters issued a statement to media announcing he had given back years of superannuation overpayments to the Ministry of Social Development. It came in response to questions from reporters.

Peters had, since 2010, accidentally been receiving a single person's pension, despite living with his long-time partner Jan Trotman. Peters said he had repaid the money immediately after the error was raised with him.

It was later reported the payments added up to about $18,000.

National Party deputy leader Paula Bennett is among the defendants in the case and will give evidence. Photo / Mark Mitchell
National Party deputy leader Paula Bennett is among the defendants in the case and will give evidence. Photo / Mark Mitchell

In preliminary hearings, the court was told by Peters "an error had been made when the form was completed".

"The plaintiff's analysis is that the MSD processed an incomplete form and made the mistake so that it was the MSD who made the calculations, not the plaintiff," Chief High Court Justice Geoffrey Venning said in a decision in October.

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Peters' lawyer, Brian Henry, says when he makes his opening statement on Monday, he intends to explain it was a mistake by ministry staff, not Peters, that led to overpayments.

Henry would not elaborate, but said the issue would be canvassed extensively.

Lawyers for the government departments are also understood to be calling staff who directly dealt with Peters' case, potentially clarifying the situation.

Court documents earlier showed Peters was also sent a letter four years into his seven years of overpayments by officials asking him to confirm if he was "single".

"I have no recollection of receiving the same but I do not doubt I would have received it," Peters told the court.

But he said because nothing had changed in his living situation from 2010, he would not have thought there was a reason to correct it.

Meanwhile, Peters' lawyers are seeking up to $450,000 in damages and declarations his privacy has been breached.

They are not, however, pointing the finger at anyone in particular.

"It's a group of people had the private information, and they were a confined and contained group of people," Henry said.

A total of about 40 people are understood to have been made aware of the superannuation issue before ministers were briefed.

Winston Peters is bringing the lawsuit against former National Government ministers Anne Tolley (pictured) and Paula Bennett, the Ministry of Social Development, the chief of the public sector watchdog, and a top civil servant. Photo / File
Winston Peters is bringing the lawsuit against former National Government ministers Anne Tolley (pictured) and Paula Bennett, the Ministry of Social Development, the chief of the public sector watchdog, and a top civil servant. Photo / File

The Ministry of Social Development is one of the five listed defendants – representing its staff.

Other defendants who will take the stand this week are National MP Tolley, who was Social Development Minister when officials discovered the overpayments, and Bennett, who was State Services Minister.

Both were briefed by officials about Peters' superannuation payments in 2017 under the "no surprises" rule – a convention that tells public servants to brief ministers urgently on matters of significance to their portfolios.

Tolley and Bennett have denied having anything to do with the leak and are expected to tell the court they knew nothing about it.

Peters' team will argue the officials breached privacy by telling the ministers.

Other defendants to take the stand will be State Services Commissioner Peter Hughes, whose office – the watchdog for the public service – briefed Bennett, as well as former MSD chief Brendan Boyle, whose agency handled Peters' superannuation applications.

For his part, Peters has subpoenaed NewstalkZB political editor Barry Soper to give evidence in the case, and Trotman will also appear as a witness, along with two unnamed public servants.

With Prime Minister Jacinda Ardern out of the country on Monday, Peters would have been in charge of the Government's weekly Cabinet meeting, but has turned the reins over to appear in court.

He lodged the paperwork for a lawsuit over the leaks the day before the 2017 election, and initially was also looking to make claims against a number of other National Party figures, including former Prime Minister Bill English and journalists. Several defendants were later dropped.

Leaked internal emails among NZ First members this year showed some were angry Peters had taken up the lawsuit against National members in the lead-up to Government coalition negations.