NZ First leader Winston Peters has lashed government ministers and public servants, saying legal subpoenas would force the truth over the leak of his pension overpayments.

Asked if the National Party could be trusted, he said: "I'll leave that to the public to decide but you've got people who engage in chartacter assassination for political reasons and its dirty and its illegal."

Peters was speaking in Whangarei this morning shortly after it emerged that his personal information had been passed to Social Development Minister Anne Tolley, deputy Prime Minister Paula Bennett and the Prime Minister's chief of staff Wayne Eagleson.

"New Zealanders should be seriously afraid of a country that's developing that sort of a characteristic, politically speaking."

He referred to Bennett having previously released personal information about a beneficiary.

"That's why New Zealand should be afraid. That's the kind of ethical standard some people think should be kosher for New Zealand."

Peters has come under scrutiny after it emerged he had to repay thousands of dollars to the Ministry of Social Development because his pension was set at the wrong rate. It had emerged he was being paid a single person's rate when he had been living in a de facto relationship.

Details were passed to media outlets who published on Sunday evening and Monday morning - but it then emerged the details had been earlier passed to Bennett, Eagleson and Tolley through the MSD and the State Services Commission.

The basis for the information being passed was the "no surprises" policy, which is intended to provide ministers advance warning of potentially significant events in their portfolios.

Peters said it was an inappropriate use of the policy and that "National MPs" knew before he did.

"That's absolute nonsense. [The policy] is to do with political administration. This is about an Opposition MP. What part of my operations have anything to do with a government MP? This is just a cover up, a camouflage, for illegal, illicit, dirty, underhand politics."

He said he had no confidence in an inquiry by MSD or SSC into the leak but promised he would get to the bottom of it through "legal" means.

"When you start spraying subpoenas around they'll be turning up and they'll be on oath and then we'll see what's gone on here."

Peters said the overpayment was an "innocent mistake" and had been resolved as soon as it was discovered.

"You've got an attempt to character assassinate someone on the basis of an innocent mistake. It's dirty, it's filthy and you work out yourself what it means when it comes to a question of trust."