National leader Simon Bridges says police know the identity of the person who messaged him claiming they had leaked his expenses information but they would not divulge their identity.
Mr Bridges confirmed that he received an anonymous message last Thursday from a person claiming to have leaked the information.
The author had also said that they suffered from mental health issues and "significant harm" would come to them if the inquiry into the leak continued, Mr Bridges said.
The opposition leader said he had informed the police of the message on Friday after receiving "significant expert mental health advice on how to proceed with this very concerning matter".
"My utmost concern was for the wellbeing of this person," Mr Bridges said.
On Sunday evening, police contacted Mr Bridges saying they knew who the person was and that they were receiving treatment for their mental health issues.
Police said there was no safety issue for the person.
"I asked specifically in relation to Mr Mallard's investigation what the position would be there if it proceeded and at no time did police tell me that the investigation should not proceed," Mr Bridges said.
"They made it clear in fact that they were working on the assumption it would and if that was so, they would inform the person in question about that to ensure they had any specific help and assistance and support that they would require."
Mr Bridges said police would not reveal the identity of the person despite the opposition leader asking.
"They gave me nothing from which I could deduce who the person was, their view was unless there were specific overriding factors such as safety, they were not going to disclose that in the interests of privacy," he said.
"There's been some comment about potentially this person identifying that they…specifics about them being in the National Party caucus.
"There is a range of credible scenarios that really mean I cannot say whether they’re a National MP, another MP, someone in parliamentary services or indeed wider than that.
"Ultimately, I don’t know who this is."
The message, which RNZ has not seen , detailed a number of conversations and information from National caucus meetings over a period.
The self-proclaimed leaker said they had done so because Mr Bridges was “arrogant” and they disagreed with his leadership style.
The person also wanted him to be held to account for his spending of taxpayers’ money.
Earlier this month, Mr Bridges stood by his MPs, saying he doesn't believe one of them leaked his travel expenses to media.
The National leader clocked up roughly $84,000 travelling around the country in a Crown limousine between April and June, as part of a national roadshow.
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