Winston Peters tells a crowd in Dunedin about an exchange he had with RNZ's Guyon Espiner on Morning Report.

New Zealand First leader Winston Peters has clashed with RNZ host Guyon Espiner in a testy - and at some times bizarre - interview.

The 25-minute interview on Morning Report on Thursday was fiery throughout, with Espiner trying to establish early on how much NZ First's policies were expected to cost.

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STUFF Trading blows: Winston Peters goes head-to-head with Guyon Espiner.

After some back and forth. Peters put it at $10 billion over 7-8 years "if you're talking about investment and borrowings".

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Espiner asked about NZ First's policy of taking GST off food. Peters corrected him: "No, off basic food".

RNZ New Zealand First leader Winston Peters joins Morning Report for an extended interview as part of its Election 2017 special interview series. As in past elections, Peters could be 'kingmaker' or 'queenmaker' if his party has enough votes to make a coalition with National or Labour. He has also confirmed he would only make a coalition with Labour if he is told what the party's tax plans are.

"It's a huge difference. You see, off food you get a huge bill. Off basic food you're talking somewhere in the zone of about $6-700 million," Peters said.

Espiner said NZ First's website had it as a $3b policy.

"It might say it on the website. It should have been corrected," Peters said.

"Oh come on, come on mate," Espiner said. "How are voters supposed to know when they look at your website and they see it there, and you're saying it's wrong?"

"I admit it's a mistake. In fact I had a discussion with my team just about two days ago about correcting that because they said, 'is it on food, and I said no, it's on basic food'," Peters said.

"This is laughable isn't it? You're asking for full disclosure from Labour and criticising them, and you can't even get your own numbers right," Espiner said.

"I'm giving you full disclosure right now. Parties make mistakes. In this case it's been corrected. It should have been corrected." Peters said.

Espiner: "So you got it wrong by a factor of five, how's that?"

Peters: "No sorry, somebody in my team got it wrong. All my speeches say that."

Espiner asked about NZ First policy to ensure most of the food an average family puts in their supermarket trolley each week will be GST exempt.

"Basic food, yes," Peters said.

"Well it says most of the food an average family puts in their supermarket trolley," Espiner said.

"I'll say it for the umpteenth time, basic food ... If you have difficulty understanding it ask your grandmother."

After Peters said the website had it wrong the website was quickly changed.

An entire section on removing the GST from food - which included the $3b costing - was removed and replaced with a single line: "Remove GST from basic food items."

Early in the interview, Peters and Espiner had a debate about how Peters' three times in Government had ended. The upshot of the conversation was that Peters' colleague was asked to go to their car to get a letter sent by then-Prime Minister Helen Clark to Peters in 2008.

"How come you've been sacked three times as a minister?" Espiner had asked.

"Excuse me. I didn't get sacked three times," Peters responded.

Espiner: "You did, you never made it a full term."

Peters said he had walked out the second time, then continued: "The third time, can I just say, I've got a document in my bag downstairs that says what you just said is a lie."

Peters' unseen colleague returned with the document about three minutes from the end of the interview.

Reading the letter, Peters said: "This letter confirms the arrangements we made on Friday, following your offer to stand aside from your portfolio responsibilities while the Serious Fraud Office makes its investigations."

"Investigations, I might add, which were found to be groundless," Peters said. "There it is, the prime minister at the time, so please don't go and tell people that I was sacked. I never was."

Peters handed the letter to Espiner, then shortly after took it back, saying "can I have it back."

Espiner: "You can actually, can I have a copy?"

Peters: "No you can't."

Espiner's response was a plaintive "oh".

With time rapidly running out, the topic turned to what might happen if NZ First was involved in talks about forming a government.

Espiner asked Peters: "What is it you want?"

Peters: "What do you mean? What do you mean by a statement like that, 10 days out from the election?" ... "Define what do you mean, what do I want."

Peter then appeared to briefly break into song: "what do you need, what do you want, what do you get?" Then he said: "This is pathetic" He then talked about having full consultation within the party "before somebody steps up like some dictator and says, 'I want this and I don't care what you guys want'."

Espiner: "Do you have bottom lines?"

Peters: "No you write them for me because I don't use the word bottom line."

Time ran out and as the pips sounded both men were smiling.