National Party finance spokesperson Paul Goldsmith says the '$4 billion hole' found in its fiscal and economic plan is an "irritating mistake".
Labour's finance spokesperson Grant Robertson said earlier on Sunday National had left a "$4 billion gap" in its economic plan after the party used the "wrong numbers".
National's plan, which includes a temporary tax cut, promised to cancel Super Fund contributions for the next decade so debt can be paid down faster.
The error occurred after National used May Budget figures instead of numbers from last week's Pre-Election Economic and Fiscal Update (PREFU). Treasury revised the size of Super Fund contributions over the next decade from $19.1 billion to $14.8 billion in the PREFU - a difference of $4.3 billion.
"As Grant Robertson helpfully pointed out, we based our fiscal plan on contributions to the Super Fund based on Budget figures, and turned out there was a slight change with the PREFU figures," Goldsmith told reporters on Sunday.
"The result of which is that instead of National's plan [of] getting net debt as a country back to 35 percent of GDP in 2034, we go to 36 percent of GDP."
He said he's apologised to National leader Judith Collins for the error.
"This is an irritating mistake, we missed it, and our external checker missed it as well and that's a mistake that we made."
"We make mistakes from time to time. This is not one that affects the critical nature of the tax relief that we're offering and also the ability to deliver good quality public services."
The error doesn't affect the temporary tax cuts, which are set to cost about $4.7 billion. This amount is going to be drawn from the COVID-19 Response and Recovery Fund.
Robertson said the error begs the question of whether there are more mistakes in National's plan.
"Not only is National's proposal irresponsible when New Zealand needs stability and certainty, they are showing they lack the experience to run the economy. There is no John Key or Bill English there anymore. No one who knows how to run a Budget would have made a basic mistake like this."
But Goldsmith said it's the only mistake in the party's plan.
"There is, in effect, a whole Budget being put together in two days since PREFU and we got one number wrong. We acknowledged that, we based it on the Budget figures not on the Pre-Election Fiscal Update figures."
Robertson labelled National's policy as "desperate, irresponsible and unaffordable".
"This is a time for stability and certainty, not policy made up on the fly that does not add up."