National deputy leader Nicola Willis said Finance Minister Grant Robertson “probably needs a cup of tea and a lie down” after an excoriating radio interview in which Robertson repeatedly called her a “liar”.

Allegations of lying are serious and difficult to substantiate - they occasionally lead to defamation action being taken against the accuser, but Willis said she would not be taking any legal action against Robertson.

“The last thing he needs is a lawyer letter. He probably needs a cup of tea and a lie down,” Willis said.

A defamation action might also put the spotlight on how much evidence Willis has for her own theory that Robertson has fallen out with the Prime Minister. So far, she has only put it forward as a theory rather than a claim for why Labour made a mistake in its GST policy.

In his regular weekly interview on Newstalk ZB’s Heather du Plessis-Allan Drive, Robertson was asked about the mistake in Labour’s GST policy that saw the first version of the policy cost $235m less than the final version.

The error appeared to come down to the fact the final policy is slated to begin on April 1 next year, but the previous version was costed as if to begin in October 2024. Willis said this discrepancy could be explained by a rift between Robertson and Prime Minister Chris Hipkins.

Hipkins himself acknowledged that the Government “did look at a number of different scenarios, calculated out the costs of a number of different scenarios, and clearly the wrong one was used in that first fact sheet”.

When asked whether he had a “disagreement” with Hipkins about when the policy should trigger, Robertson said any suggestion of a rift was a “complete lie”.

 “Complete lie, Heather, why would you repeat a lie that Nicola Willis has told with no evidence?”

Robertson repeated the “lie” allegation multiple times.

“That is a lie - she is lying about that. You get her on and ask her what the evidence is for that,” Robertson said.

“Is that how it works now, people just get to make something up and without checking with anybody it is just asserted - is that how it works now?” Robertson said.

Du Plessis-Allan cut Robertson off, saying that “you [Robertson] have said that without us knowing that you were going to say that - so if there is a defamation case it is you and not me”.

Willis did not furnish any evidence for the disagreement claim beyond the fact that it would explain how the mistake had been made.

“I pointed out that it would be explained by them, having initially looked at a later date, I was clear that that was my opinion and that you needed to ask him for an explanation.

“I’m yet to hear a real explanation from him to be accused of lying is completely over the top,” Willis said.

Robertson repeated the claim on his way into his caucus meeting.

“There was a statement made yesterday there that there’d been a meeting with a row and an argument between me and the prime minister about the timing of GST, which is completely untrue,” Robertson said.

Robertson said he was frustrated by the opposition tactic of throwing out allegations that the Government is forced to deny.

“I’m frustrated by the form of questioning that says,’ here’s an accusation that has no evidence behind it - what do you say to that?”

“Then the politician says, well, that didn’t happen and then the story is written that the politician denies it. To me, that’s a very short-sighted and not particularly useful,” he said.

Willis has certainly not held back from making allegations against Labour recently. She has a fairly good success rate, having leaked the party’s ditched wealth tax plans, and the fact the party’s election tax policy was to take GST off fruit and vegetables.

She also alleged that the Government’s books were in such bad shape public service chief executives had been instructed to “batten down the hatches and desperately dig up major savings to help cover-up a growing crevasse in the government books”.

While Robertson has admitted he met with public service chief executives and that revenue receipts were coming in worse than forecast, he said that discussions about finding savings from the public service was part of an already-announced initiative contained in the most recent budget, and not an emergency measure in response to a weakening fiscal position.

Meanwhile, Hipkins pointed the finger back at National, recalling its own recent caucus disagreements.

He said there was no show down between himself and Robertson and that the “only recent example I can think of where a party leader has embarrassed their finance spokesperson was when Christopher Luxon overruled Nicola Willis on housing.”