New Zealand First has upped the ante in the saga over its mysterious foundation, with party leader Winston Peters' long-time lawyer Brian Henry threatening to sue National for $30 million.

In the House, senior National MP Nick Smith tabled a letter in which Henry issued a clear ultimatum to the veteran MP.

"Repeat what you said in the House in public or apologise."

This was in relation to comments Smith made about the transparency of the New Zealand First Foundation, and questions about party donations.

Henry said in the letter: "Please note if you oblige with this request [to repeat the comments outside the House] I will sue you for defamation for general damages together with special damages which from the consequences of your and Mr Espiner's [RNZ reporter] action could be as high as $30,000,000."

A spokesperson for National said the party would not be commenting on the letter or the threat.

But speaking to Newstalk ZB, Smith said he stood by the comments he made in the House.

Prime Minister Jacinda Ardern and Deputy Prime Minister Winston Peters under fire from the Opposition, on the NZ First Foundation, during Question Time in Parliament. Photo / Mark Mitchell

He would not, however, repeat the comments he made in the House under privilege for "obvious reasons".

Smith referred Newstalk back to the comments he made in the House on the issue which he said he stands by.

But Smith would not apologise either – "I stand by the statements that I made in the House," he said, when pressed repeatedly.

Speaking to media not long after the letter was tabled, Peters said he wasn't suing anybody – "at this point in time".

He directed all questions on the issue back to Henry.

Henry did not respond to the Herald's request for comment.

When pressed about Henry's letter, Peters said he had "no idea what [reporters] are talking about".

"You're asking me about an action, details of which I'm not privy to and I think you should ask the person bringing the action, not to come to someone who is not a source at all."

Peters today leaves for Japan until Sunday for the G20 meeting.

Prime Minister Jacinda Ardern was also reluctant to comment on the issue.

"That's simply not a matter for me," she said and, like Peters, said that was a matter for Henry.

"Any decision that a private citizen like that chooses to make in relation to a political party, that's for them."

Henry's letter to National comes as questions regarding the transparency of the New Zealand First Foundation and donations to the NZ First Party continue to loom.

Stuff reported that the foundation appeared to have hidden political donations worth almost half a million dollars between April 2017 and March this year.

Many of these apparent donations to the foundation do not appear on the party's electoral returns.

In the House, senior National MP Nick Smith (left) tabled a letter in which Henry issued a clear ultimatum.

Stuff also reported that Tauranga-based MP Clayton Mitchell was responsible for handling any larger donations to the party.

He was not in the House today and has not responded to the Herald's request for comment.

No other NZ First MPs who were in the House were commenting on the issue either.

Simon Bridges and Nick Smith have been invited to either repeat their claims outside the House or apologise. Photos / Mark Mitchell

Peters was also not commenting on this.

But he revealed that the New Zealand First Foundation has volunteered to provide written material to the Electoral Commission and to meet with officials about some of the issues raised in the media.

He also said he had learned that overnight, New Zealand First has had the biggest surge in volunteers and membership applications, since 1996.

"And I thank you all, my friends in the media for making that possible."

This comes as former NZ First Presidents and executive members have revealed to the Herald they were in the dark over the NZ First Foundation.

Kevin Gardener – who was party president from 2010 to 2013, said: "I don't know a damned thing about it".

Lester Gray – who resigned as NZ First president in September after refusing to sign off financial reports for "moral" reasons – also denied knowing about the foundation.

The NZ First Foundation was "nothing to do with my role as the president or the NZ First board," he told the Herald.