New Zealand First is reporting a breach of party member details to the police and the Privacy Commissioner.
A database including the names, occupation, phone numbers and addresses of members and people affiliated with the party has been leaked to news media and opposition MPs.
It details information as recent as 2016, and includes contact details of party leader Winston Peters and his partner Jan Trotman.
In a statement, Mr Peters said "the information given to the media this afternoon is a deliberate and malicious misappropriation of data by a disgruntled souce".
"Despite its age this is a serious breach and as such is being reported to the police and the Privacy Commissioner," Mr Peters said.
It's the latest in a series of leaks that have included complaints about the 2017 election campaign and list selection, and the use of donations.
Last week, the party president Lester Gray's resigned for "moral reasons", saying he refused to sign off financial reports due to being kept in the dark on the party's expenditure and donations.