NZ First leader Winston Peters has his own lawyers investigating the source of the leaks, but says his personal views won't colour post-election negotiations.

There was "absolutely no benefit" for National to leak the details of NZ First leader Winston Peters' pension problems, says deputy leader Paula Bennett.

She has rubbished suggestions National might have seen gains if Peters' party dropped in the polls, telling TVNZ's Breakfast the theory made no sense.

"It just doesn't make sense that it was. Why would we leak something on the day of our campaign launch, which we want to be all about us quite frankly.

HAGEN HOPKINS/GETTY IMAGES National deputy leader Paula Bennett has rejected suggestions that a leak of Winston Peters pension problems would benefit her party. "It just makes no sense".

"We held the information confidentially - I just have absolutely no doubt but to believe my colleague Anne Tolley. I know I didn't leak it anywhere else, so there was effectively three people that knew and only three people," she said.

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"I know that we didn't talk to anyone, and as a consequence of that I don't think it was within the National Party."

It comes after three separate investigations were launched yesterday, into the source of a leak to journalists about an error in Peters' pension application, that resulted in him being overpaid for a number of years.

As soon as Peters was alerted to the error he repaid the money within 24 hours. He won't reveal the size of the sum, but rejects it it was as much as $18,000, which some media have reported.

Inland Revenue, the Ministry of Social Development and Ministerial Services have all launched internal inquiries into the handling of the information, and State Services Commissioner Peter Hughes said if a leak in the public service was discovered, it would be met with "zero tolerance".

Prime Minister Bill English said as much yesterday, but he had no reason to believe the leak came from any of the three people in the Beehive who knew; Bennett and Social Development Minister Anne Tolley were briefed by their officials after the matter was resolved, and Tolley briefed English's chief of staff Wayne Eagleson, who made the decision not to brief the Prime Minister.

English said on Tuesday it would have, in fact, been better if none of his ministers were briefed on it.

On Wednesday, Bennett rejected a line of questioning that seemed to suggest National would benefit if Peters went down in the polls, however National could find itself on the back foot regardless when it came time to sit around a negotiating table with Peters.

The NZ First leader has confirmed however, that any decision to support a party into Government would be a caucus decision, and his personal views would not be a factor.

"What you're saying is ... that in the interests of the nation and its economic and social survival that one should put his personal views above the serious decision one's got to make, and above that of his caucus and how they see it ... and I'm not going to do that," he said on Tuesday.

Bennett rejected the idea that such a leak would end up in National's favour, if support was peeled off NZ First.

"It's just not something that we'd do and I can't see benefit for it, because this is probably exactly where it would end up and we just wouldn't have done it, and we didn't do it."

Bennett said she imagined there were a number of people in departments and Peters' own party who knew the details of the matter. However, she said she was not pointing the finger at anyone, or making assumptions.

She mocked questions over who might have had a journalists' phone number.

"What, did we put on a pretend voice to ring journalists? It didn't happen," she said.

"You're jumping, as Winston Peters is, to a really long bow; that our integrity and our role as ministers in holding confidential information is absolutely pivotal to our roles.

"It's something we're used to doing and I can tell you from my perspective, I just knew this was about an opponent, I didn't feel comfortable in it, so I chose to not tell a single soul."

Bennett has confirmed she did speak about it with Tolley, because she was aware Tolley had already been briefed by her officials.