The error is in regards to the "savings" National would make by cancelling its contributions to the NZ Super Fund.
National had used the Treasury's budget update to calculate that the party would "save" $19.1 billion by scrapping contributions to the NZ Super Fund.
But, National should have been using the Pre-Election Economic and Fiscal Update (Prefu) – which showed that the "saving" would have been $15 billion over 10 years.
"It changes the debt target in 15 years by 1 per cent," Goldsmith said, before turning his focus on Labour.
"Labour have quietly cut the Super Fund to reduce debt, which they didn't announce and we missed it. We are surprised, given their criticism of us for suspending payments to the Super Fund.
Despite his apology for the mistake, his leader Judith Collins was playing it down.
Speaking to media she dismissed the blunder as "a little error," "entirely inconsequential" and something "not many people are worried about".
"Nothing Grant Robertson says worries me."
But Robertson, Labour's finance spokesman, said the mistake was a blow to National's credibility.
"National has to be responsible for their mistake; it is a $4 billion mistake in their economic plan – they now have to answer the question of whether there are other errors in this plan," he said.
"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."
The accusations of accounting errors strike a remarkably similar tone to similar allegations made in the 2017 campaign, only the roles were reversed.
Back then, it was National's finance spokesman Steven Joyce accusing the then-Labour opposition of having an "$11.7 billion fiscal hole".
Finance Minister Grant Robertson. Photo / Mark Mitchell
Criticism over the credibility of spending plans bears a striking resemblance to the pre-2017 election debate over what National alleged to be an $11.7 billion fiscal hole.
