National Party leader Simon Bridges wants Prime Minister Jacinda Ardern to take stronger action over allegations that New Zealand First has breached electoral law.
The party has received tens of thousands of dollars in loans from the New Zealand First Foundation. Loans are not subject to the same disclosure rules as donations.
The Electoral Commission has received a complaint and is seeking more information from the party.
Simon told RNZ's Morning Report he thought the passing the matter to a Commission investigation was "good" but "not sufficient".
"I think what we first need to see is leadership from the prime minister, her stepping in, ensuring there's a full independent investigation," he says.
When challenged that the issue being before the Electoral Commission meant there a full and independent investigation was already under way, he said it "goes to the heart of the government in my view and its integrity".
"New Zealand First props up the government. It is at the core of this government. The prime minister may want to wash her hands of it, she can't."
"She should be asking what other steps should or shouldn't be taken. The reality is she isn't".
Interviewer Kim Hill pressed Simon on what Prime Minister Jacinda Ardern should do differently.
"Why is letting the Electoral Commision investigate not doing her job?" she asked.
"It's good, okay," he responded. "I'm not making any claim that is anything other than good. The Electoral Commission looking at this is good".
"How about this - even if the Prime Minister were today to come out today and say 'you know what I've sought advice from my hundreds of advisors in Crown law and DPMC and PMO, we have gone through this, actually there's serious allegations made but it is now with the Electoral Commission, they have the powers to refer it to police etcetera etcetera', she won't even do that.
"She's washed her hands of it, she's sitting it out. She does not get to."