National Party leader Simon Bridges says Prime Minister Jacinda Ardern must "lead" and ensure all steps are being taken to investigate claims that New Zealand First hid donations.
A story released by Stuff on Tuesday 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. Documents allegedly showed the donations total more than $320,000.
Law expert Graeme Edgeler has questioned the donations' legality if hidden.
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.
But party leader Winston Peters says all money received from the New Zealand First Foundation has been paid back and he is confident the law has been followed.
The Electoral Commission told Newshub on Tuesday it hadn't been shown any of the documents referred to by Stuff. It said it would be contacting New Zealand First and the foundation for further information.
In a statement, Peters said he would cooperate.
"I am confident that New Zealand First has operated within electoral laws, now and for the last 27 years. Declarable donations were declared to the Electoral Commission."
Bridges says if the story is correct, it would be the largest electoral law breach in history.
"If the facts we have seen in the stories are correct and made out, what you have is a long-term, systematic, purporting of election laws," he told The AM Show on Wednesday.
The National Party leader says Peters won't talk about the allegations other than by releasing his statement. He now wants the Prime Minister to act and ensure every step is being taken to investigate the claims.
"Winston Peters won't talk about it… and then you have got a situation where the Prime Minister is trying to wash her hands of this. She can't," he says.
"It goes to the heart of her Government's integrity and the party that props up the Government. She needs to lead on it and she needs to ensure there is a full independent investigation going on."
Ardern said on Tuesday that she wouldn't be drawn on whether any law had been broken as that was something for the Electoral Commission to determine. She did, however, rule out calling for a snap election, as some commentators have suggested as a possible means to rid her Government of New Zealand First.
Deputy Prime Minister Peters did speak to media on Tuesday about the allegations, including MagicTalk. He told Peter Williams that donors were entitled to privacy.
"If they're making donations to the party legally, they are entitled to the protection of the law," Peters told 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."
He also pointed to allegations raised by former National MP Jami-Lee Ross last year against National leader Simon Bridges.
Ross accused Bridges of committing electoral fraud by asking him to split up a $100,000 donation from Chinese businessman Zhang Yikun into smaller amounts, so they could be hidden from the Electoral Commission.
Bridges said on Wednesday he was cooperating with authorities over that matter and was sure he would be vindicated.
On whether the concerns raised on Tuesday affected National's willingness to potentially enter into a coalition with New Zealand First after next year's election, Bridges said they did weaken the chances of that. However, he wouldn't completely rule it out.