New Zealand First has admitted someone within the party made a mistake by not declaring $50,000 in donations from the Spencer Trust to the party in 2005.
It comes as the Serious Fraud Office and the privileges committee are investigating the party's money.
It was a document obtained exclusively by 3 News yesterday that showed the Spencer Trust paid $50,000 to NZ First in 2005.
But it was never declared, and that broke the law.
Today the party admitted it was a mistake, but was not initially willing to talk about it.
"No comment," said MP Doug Woolerton.
NZ First deputy leader Peter Brown came clean about the donation, but put it down to human error.
"There's always concern when people make mistakes, but we're all human and those sorts of things do happen," says Brown. "It probably couldn't have happened at a worse time, but it has happened."
John Key has already distanced himself from party leader Winston Peters and NZ First, ruling out working with either after the election. He says Peters has to first explain the undeclared donations.
"It would appear that New Zealand First is in breach of the Electoral Finance Act, and that's one of a number of allegations that will now have to be cleared up."
Helen Clark was also feeling the heat. She stood Peters down last week because of the Serious Fraud Office inquiry.
She is doing her best to distance herself from Peters and his party's problems.
"From my point of view as Prime Minister, Mr Peters has stood aside. I'm getting on and doing the work and I've got other things to do," she says.
Brown now says the party will produce documents that prove it was a simple oversight.
"We have documented evidence proving that it was a genuine mistake. People do make mistakes."
So New Zealand First survives this one for now. Technically it has broken the law, but can not be prosecuted.