Deputy Prime Minister Winston Peters has taken to the witness stand in the High Court to accuse his political opponents of deliberately smearing him by releasing his pension payments.

Peters also said it was not his fault his superannuation had been overpaid for seven years, because the Ministry of Social Development had incorrectly processed an incomplete application form.

And the overpayments, which public sector heads were alerted to in 2017, should never have been passed to National Party ministers, he told the court.

"It wasn't controversial, it was an operational mistake," he told the High Court in Auckland today.

Peters' privacy case against former National Party ministers Paula Bennett and Anne Tolley, government department heads and public servants began today. It relates to the leak of his pension overpayments in the leadup to the 2017 election.

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 t