New Zealand First leader Winston Peters says he received a superannuation overpayment because he was on a single person's rate while he was living with his partner.
Mr Peters revealed last night his fortnightly pension had been overpaid for several years, and that when he found out last month, he gave the money back within 24-hours.
He told Checkpoint with John Campbell he did not know how he ended up on that rate.
"I turned up and made an application with my partner, with a very senior person at MSD and they gave me advice as to what I'd get.
"I am not the absolute expert on these sorts of things, but I thought it was about right because it looked much lower than the standard and they apparently gave me one rung too high and as a consequence it wasn't discovered."
Mr Peters said neither he nor the Ministry of Social Development (MSD) could figure out which party had made the mistake.
"We sat down and tried to work out how we made this mistake, I have to tell you that it's not rare at all, but as a consequence we decided we couldn't discover where the fault lay.
But he said he did not request a breakdown of the overpayment - just the total amount required to clear the debt.
"At no point have I been given all the calculations, I didn't ask for it.
"I said: I want you to make as the number one job that you've got here on this case, to finalise what you think the total amount is, interest on top of that, and send me the information, and I will deal with it."
Mr Peters is refusing to give a privacy waiver enabling the ministry to discuss any details of the overpayment.
National Party leader Bill English said it was a matter for Mr Peters.
"Because this is about his personal finances and his personal circumstances, clearly there's questions and I think that a lot of people will be asking them, but it's up to him to deal with that."
Mr English was asked whether he would reconsider working with Mr Peters.
"You have to see what the facts of the matter are.
"Mr Peters believes it's a matter that's settled, that MSD think it's settled so if there are any more questions to be answered they'll get answered and we'll just have to see how the facts unfold."
Labour leader Jacinda Ardern said it was a different situation from Metiria Turei, who resigned as Greens co-leader after admitting to benefit fraud.
"In that circumstance she ruled herself out ... it would be helpful if we had more information but still ultimately this is an issue for Mr Peters to be answering questions on, not the Labour Party."
But ACT leader David Seymour said it was a "debacle".
"And there's a lot more to come because with Winston Peters there's always more and it's always worse than initially thought," he said.
"I just wish that he's put the paper out and prove that it really is a simple administrative error."
The situation facing Mr Peters highlighted bigger problems with the system, said the Opportunities Party leader, Gareth Morgan.
"Doesn't the fact he didn't even notice he was getting, you know, paid just tell us how broken our benefit system is - you've got benefits here going to people who don't need them."
In a statement, the Ministry of Social Development said while it recognised the public interest in the case, it "holds a great deal of very personal information" and it has a duty of care to protect.
"It is not our information to share.
"As stated by Mr Peters, we have been in contact and the matter has been resolved to the Ministry's satisfaction."