Leaders debate: Ardern and English clash over Budget 'black hole'

National's claim that Labour had made an $11B error in its fiscal plan is "an absolute fiction" and an attempt to attack Labour's credibility, Jacinda Ardern said at the Newshub Leaders Debate on Monday night.

But Bill English stuck by National's claim, saying Labour's numbers didn't add up and they would have to introduce new taxes to make up for the hole.

Host Patrick Gower first asked Ms Ardern about National's attack on Labour's books.

"Paddy, that is an absolute fiction. We've had Brian Fallow, Bernard Hickey, Vernon Small, and an independent economist BERL, who verified our fiscal plan, all come out and say what National said was a complete fiction," Ms Ardern said.

"I want to actually address the elephant in the room. I know what National are trying to do here is damage Labour's economic credibility, even though we ran nine budget surpluses, we had the strongest continuous economic growth since WW2, we got net Crown debt near to zero, we set them up for a good run when the GFC hit, and despite that, I know they are trying to hit our credibility."

But Mr English said Labour's numbers didn't add up.

"This is why: when you look at how they've calculated things ... they've said, let's say the police get a pay increase. What they've said is they've got enough money to pay it for one year, but not for the years after."

Ms Ardern interjected saying that was "not true".

Mr English said: "So they haven't rolled it out, and that means the effect of it is while they've allocated money to health and education, they haven't got any left for anything else.

"Now the Government runs security services, it runs defence forces, it runs police, it runs services for vulnerable children. There's nothing in the Labour budget for those."

"They have to fill the gap either with higher taxes, which I believe would now be inevitable under Labour or borrowing."

Ms Ardern said Labour have "based these figures on exactly what the Government books have said themselves. If it’s not in there it means you haven’t budgeted for it either," she said.

"That is simply wrong," Mr English said.