The National MPs deny leaking the information, which was provided to two media organisations by anonymous sources.
The court is considering not only at how the information was made public, but also how Peters came to be overpaid in the first place.
Giving evidence this morning, Peters said he was not to blame for the overpayments. He was "perturbed" by the error which led to him being paid a single person's pension despite being in a long-term relationship with partner Jan Trotman.
When Peters first applied for superannuation at a Ministry of Social Development office in April 2010, he said he had Trotman with him and introduced her as his partner.
He did not complete one of the questions in the application form - which related to whether he had a partner - but the form was processed by MSD anyway. His pension rate was set based on this incomplete form.
"I could not accept in any way that the error was of my doing," he said.
Twenty-three pension applicants made the same error on their application form that month, Peters said.
When the error was picked up by MSD in 2017, former MSD head Brendan Boyle was informed. He sought advice from State Services Commissioner Peter Hughes.
Those two public sector bosses should never have told National Party Ministers about the issue, Peters said: "It was an operational matter and no more."
The "no surprises" convention under which Bennett and Tolley were told about the overpayments should not be used for gossip or trivia, Peters said.
The leak in 2017 which showed he had been overpaid by about $18,000 was "slanderous" and was still damaging his reputation two years after the event.
"It is still brought up by my detractors as a slur to this day," he said.
'HIGHLY OFFENSIVE'
A leak which showed Winston Peters had been overpaid for his pension was a serious "slur on his reputation" in election year, his lawyer says.
The Deputy Prime Minister's lawyer Brian Henry said in the High Court today that the leak of his pension details in 2017 suggested - at worst - that Peters had committed fraud.
That was as damaging to a person's reputation as an allegation of sexual offending, Henry said.
"People were running around town saying that Peters had somehow chiselled the MSD for his benefit."
Peters' privacy lawsuit against former National Party ministers Paula Bennett and Anne Tolley, the Ministry of Social Development, the chief of the public sector watchdog, and a top civil servant, began today.
Peters claims that the ministers and public servants were responsible for media finding out that he had been overpaid for his superannuation for seven years, which he later paid back. The alleged leak occurred in the lead up to the 2017 election.
In opening the case today, Henry said the court would have "no difficulty" in finding that Peters had a reasonable expectation of privacy in relation to this information.
A reasonable person would also find it "highly offensive" that the information was made public, Henry said.
Bennett and Tolley were told about Peters' pension overpayments under the "no surprises" convention, which requires public servants to tell their respective ministers about significant issues in their portfolios.
But Henry said the information was not something a minister was entitled to know about.
"It was a matter in respect of an individual, that is the sole discretion of the public service."
He said information, after being passed on to ministers by public servants, made its way into the media via anonymous sources.
"A chain has been created, and it has ended up in the hands of someone malevolent, and that has ended up putting a slur on the event."
The leak occurred four weeks before the general election and two weeks before voting began, Henry said.
"This is about damage to his reputation."
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.
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THE STORY SO FAR
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.
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.
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 at the time.
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.
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.