Winston Peters says he won't stand-by while information about his party is leaked.
The New Zealand First leader has asked the party president to make a police complaint over what he's calling a 'massive breach' of party information.
It's believed to be related to the party's Foundation.
Peters told Mike Hosking having a suspicion is one thing, but evidence is what matters.
"What we're not going to do is standby while malicious campaigns involving theft, which is a criminal act, on disclosure which was not entitled to be in the public domain."
Peters says the person he suspects of leaking is no longer in the party, but he does not believe that person has any dirt on him.
"Proof is what's required and proof of the breach of the law is what's required, not allegations and innuendo."
The allegations have potentially had an impact on NZ First's polling numbers.
The latest Newshub-Reid Research poll shows New Zealand First below the critical five percent threshold.
It means that Labour would likely be able to govern with just the Greens' support.
But Peters says he shouldn't be discredited just yet.
"There are two parties that are going to be seriously worried about that poll, and New Zealand First is not the third party that needs to be remotely worried about it."
He says he's been written off by polls year in and year out.