New Zealand First leader Winston Peters has asked his party president to prepare a police complaint over the "massive breach" of the party's information.
"This morning I have recommended to the New Zealand First party president that she begin preparing a complaint to the police over the massive breach of New Zealand First's party information," he said in a statement on Sunday.
"New Zealand First has so far been sensitive to the circumstances surrounding the theft of party information but can no longer tolerate the mendacious attacks against the party and its supporters."
A story released by Stuff in November last year raised allegations about large donations from the New Zealand First Foundation to the political party New Zealand First not being declared to the Electoral Commission.
According to Stuff, the New Zealand First Foundation collected more than $500,000 in donations in two years.
However Stuff reported it had obtained documents showing six donations had not been declared to the Electoral Commission in 2018. The investigation also cited evidence that 13 donations hadn't been declared in 2017.
The money is said to have been used on everything from hiring boxer Joseph Parker to speak at a party conference to legal advice for an MP.
Public law expert Graeme Edgeler told Stuff the Electoral Act may have been broken because donations from the foundation had not been declared.
Peters has previously rubbished claims New Zealand First tried to hide donations from the New Zealand First Foundation, describing it as "filthy allegations without any proof".
He told Magic Talk in November all of the loans the New Zealand First Party has received from the foundation had been paid back, and he said those making the donations are entitled to privacy.
"If they're making donations to the party legally, they are entitled to the protection of the law," Peters told host Peter Williams.
"Not some snooping advice that says 'you've given $100 to the New Zealand First Party, why'd you do that?' That's not what our great society's based on.
"It's based on the secrecy of ballot behaviour including the funding of political parties. You take that away, and you've got a dictatorship.
"Everything that was required to be declared by New Zealand First for 27 years has been declared."