Prime Minister Jacinda Ardern says it would be dangerous for democracy if political parties step in as the arbiter of New Zealand's electoral law.
The National Party is demanding Ms Ardern ensure the New Zealand First donations scandal is properly investigated.
The Electoral Commission is looking into a complaint about an entity called the New Zealand First Foundation, which has loaned the party tens of thousands of dollars. Loans are not subject to the same disclosure rules as donations.
Reports on Stuff.co.nz have prompted further questions about how the foundation is being used.
New Zealand First leader Winston Peters said he was confident the party had always acted within the law and he was looking forward to discussing the matter with the Electoral Commission.
National leader Simon Bridges said Ms Ardern needed to step in and ensure the investigation was robust and carried out properly.
"It's Winston Peters that props up this Jacinda Ardern government, he is her number two, he is at the very core of it and I think New Zealanders want to know whether they can trust it.
"A question she should be able to get good advice on and answer is whether this should be referred by her or someone in government to the police," he said.
But Ms Ardern said this was a matter for the Electoral Commission to handle as an independent third party.
"It would be totally inappropriate for any political party to have a role in investigating or inquiring into another political party.
"That is not what I call a transparent democracy that New Zealand upholds, we need independence here," she said.
Ms Ardern said she had spoken to Mr Peters who assured her "New Zealand First are actively and openly" working with the Electoral Commission.
However she would not say whether she had sought an assurance from him that no donors to the New Zealand First Foundation had received any policy or funding favours.
"Ultimately those are matters for the Electoral Commission to determine and they are working with them on that.
"They're the independent third party, that's where the public look to for those reassurances," Ms Ardern said.
New Zealand First leader Winston Peters said "of course" the public could trust him, as he continued to face questions about political donations.
"We've got the Electoral Commission looking at all of those matters and when they're finished, they'll give you the answers."
The party would need "some considerable time to assemble all the information which we're happy to give to the Electoral Commission and I look forward to them telling you the truth".
A "whole lot of people" were making "fake news, false allegations - and we'll prove that to you," he told reporters.