Winston Peters now 'knows' who pension leaker is, is taking privacy case back to court

The Deputy Prime Minister says he now knows who leaked his pension details to the media - and he's going back to court to prove it.

Last month, Winston Peters' privacy claim against National MPs Paula Bennett and Anne Tolley and others was dismissed after he failed to identify exactly who was responsible for the leak.

The High Court agreed that the New Zealand First leader's privacy was deliberately breached, but his claim could not proceed as he could not prove which of them - if any - leaked his accidental over-payments just before the 2017 election.

But now he knows for sure, he says, and is appealing the High Court decision because it "didn't draw the right inferences from the facts it found".

"I am not persisting with this case just for myself, but for all people who have had their privacy breached," he said in a statement to media.

"It is seriously worth highlighting that despite there being around 51,000 people who have made a mistake on their Super Application since 1976, only a single case surrounding an individual has ever made the headlines.

"It has always been known that this case was leaked. Leaked with an intent to do harm. Leaked for a purpose."

Peters says his superannuation is a private matter and should not have been raised with ministers. Bennett and Tolley have consistently denied being responsible.

"Within Government and the Civil Service no less than 42 individuals knew about this one case," Peters' statement continues.

"The matter was resolved before it got the attention of Ministers via the Ministry of Social Development. So what was their investigation, internal inquiry and internal disclosure about?

"Was it to say that we have this case that has been resolved, but we want you Ministers to know about it? But why?"

In a statement sent to media last month, New Zealand First said the case had caused Peters "considerable stress".

"It is seriously welcome to see that Justice Venning confirmed that this was a deliberate and malicious breach of privacy done with the intent to damage my reputation and cause harm," Peters said.

"This was always going to be a difficult case because as the decision points out despite evidence of malicious behaviour we had to prove who did it.

"Our values, human rights and democracy have privacy as their foundation stone. On this matter the decision of the court was very encouraging."