National Party leader Judith Collins has finally launched her party's campaign today - but the lead-up was blighted by a torpedo from Labour.

National's original planned campaign launch had to be cancelled after the second outbreak of Covid-19 emerged just days before it.

Collins instead launched the campaign online - from Avalon studios near Wellington.

But much of her thunder was stolen by the fact her finance spokesman, Paul Goldsmith, was forced to admit his team had made a $4 billion mistake in the party's calculations.

"We make mistakes from time to time," he said, adding that "I never claimed to be perfect".

Before Collins joined the stage, National played a teaser-trailer – complete with an epic backing track and a voice over – for Collins with her talking about the current state of the country.

She called it a "bittersweet day," given the party's campaign launch had to be virtual, but pointed the blame on the Government for not doing enough to keep Covid-19 out of the country.

"I do not think the enormity of what is in front of us has yet sunk in for many New Zealanders," she said.

"The economic morphine pump is running low, and the pain is very real."

She took aim at Labour, calling its plan "short-term" and "low-value".

"Labour's erratic, unplanned governance and lazy incompetence are what we have come to expect."

She said National would leave a complete upgrade of New Zealand's transport, education and healthcare infrastructure for the next generation.

Collins teased policies she will announce this week.

"In the days ahead I'll announce policy that will do even more to springboard businesses out of these difficult times, further boosting long-term employment."

She finished by pitching directly to voters.

"Leading New Zealand through this deep recession will require a National government. One that is bold, competent, and decisive – not prevaricating and weak."

The lead-up to the launch was hit after Labour's Grant Robertson revealed a $4 billion hole in National's economic plan.

National's finance spokesman Paul Goldsmith has copped the mistake on the chin, saying it was "irritating" but did not mean the whole plan was flawed.

Speaking to media before National's official campaign launch, Goldsmith apologised for the error, calling it "irritating".

"This is an irritating mistake, we missed it and our external checker missed it as well – that's a mistake."

He said he had apologised to Collins.

"We make mistakes from time to time," he said, later adding that "I never claimed to be perfect".

"These things happen – they're irritating, [but] we move on."

The error means National's debt repayment plans have been changed, It now plans to have debt down to 36 per cent of GDP by 2034, instead of the 35 per cent it promised on Friday.

The error would not affect National's plans to offer tax cuts, Goldsmith said.

This morning, Robertson accused National of not properly costing its policies.

The error was in regards to the savings National would make by cancelling its contributions to the NZ Super Fund.

National's economic plan said scrapping these contributions would save $19.1 billion – but Robertson said Treasury's estimates show the Government would have been contributing $15 billion over that time.

"National has used the wrong numbers," Robertson said.

Goldsmith said the party had used numbers from the original budget forecasts, rather than the ones from the Pre-Election Economic and Fiscal Update (Prefu).

"It changes the debt target in 15 years by 1 per cent," Goldsmith said.

That 1 per cent amounts to $4 billion.

He went on to point out that National's error actually showed that Labour had "quietly cut the Super Fund to reduce debt, which they didn't announce and we missed".

"We are surprised, given their criticism of us for suspending payments to the Super Fund."