Winston Peters admits taking single-person pension while in relationship

New Zealand First leader Winston Peters usually has a lot to say about superannuation, but not so much on Monday, after it was revealed he'd received an overpayment on his pension for up to seven years.

On Monday evening, Mr Peters admitted to RNZ he was receiving the pension at a single-person rate while in a relationship.

Mr Peters says he paid it back as soon as he found out, but the self-styled campaigner for transparency refuses to say how much it was, instead going on the attack.

Mr Peters accused Newshub reporters of lying to the public, insisting he'd been totally honest himself.

But his story hasn't been totally consistent.

Newshub raised the allegations of superannuation overpayment on Saturday, and Mr Peters wouldn't give a straight answer.

When asked whether he would deny or rule out questions of overpayment, Mr Peters said he did not "have any comment to make about people running around making malicious statements about Winston Peters."

But 15 hours after this phone call, Mr Peters put out a press release admitting he'd been overpaid his superannuation, which he started receiving in 2010 when he turned 65.

He called it an error that had since been sorted out with the Ministry for Social Development - however he won't release the documents.

Newshub did ask MSD to release the documents, but it refused, instead saying it's a private matter. 

It's not known how the so-called 'king of superannuation' - who knows the pension system inside out - didn't notice the overpayments.

The overpayments emerged on July 15 - the day before Green MP Metiria Turei went public about her benefit fraud, which may explain why he was sympathetic as the scandal waged. 

At the time, Mr Peters said, "No one wants that to be their legacy. If it had been handled at the start properly with some sound advice then this would not have happened."

Mr Peters paid the money back almost immediately but won't say how much, instead repeatedly saying "it's a private matter".

His caginess is at odds with his usual demands for openness, transparency and accountability.

ACT's David Seymour is challenging Mr Peters to release the documents.

"If it really is just a small administrative, mysterious error, why not just release all the material and let people make up their minds for themselves," Mr Seymour told Newshub.

It brings back memories of 2008 - when Mr Peters faced questions over political donations.

Mr Peters could be king or queen maker, and there was little criticism from the potential king and queen. 

Labour leader Jacinda Ardern and Prime Minister Bill English both said it's "an issue for Mr Peters".

Mr Peters has long fought for pensioners. Now he's fighting off claims about his own pension entitlements.