On Sunday, New Zealand's opposition launched their October 17 election bid with a slick hour-long informercial-style affair.

Beamed to thousands of loyal party members on Facebook, National leader Judith Collins painted Prime Minister Jacinda Ardern's government as an out-of-control tax-and-spend administration.

With New Zealand still draped in COVID-19 related restrictions, Ms Collins was unable to hold a large-scale rally to kick off her campaign, as Ms Ardern did last month when restrictions were relaxed.

Unfortunately for National, the event was overshadowed by the emergence of a multi-billion dollar hole in their policy costings - found by Labour's Finance Minister Grant Robertson.

Opposition finance spokesman Paul Goldsmith was forced to apologise to his leader for the $NZ4 billion ($A3.7 billion) error in their economic plan.

"This is an irritating mistake. We missed it," Mr Goldsmith said, sarcastically adding "as Grant Robertson helpfully pointed out".

Mr Robertson said it showed the "a lack of credibility" in National's team.

"There's no Bill English. There's no John Key," he said, referring to Ms Ardern's two prime ministerial predecessors.

"This is the kind of mistake you'll get with that level of inexperience.

"When you're putting together these things you do have to be able to actually make the numbers add up."

The blunder came as Mr Goldsmith used an outdated costing of the government's super fund contributions, which National had downscaled from $NZ19 billion to $NZ15 billion in last week's budget update.

Mr Goldsmith played down the boo-boo, saying National still had a better plan to pay back the country's ballooning debt.

"Instead of National's plan getting our net debt back to 35 per cent of GDP in 2034, we go to 36 per cent of GDP," he said.

"This is in marked contrast with the government's plan which means we are at 48 per cent of GDP."

Despite the clumsy mistake, the show had to go on, and on it did.

Hosted at a television studio in Lower Hutt, just north of Wellington, National's launch was glitzy and hosted by a TV star in their own ranks.

Former presenter and gardening show host turned conservative MP Maggie Barry held court, introducing candidates from across the country.

A cross to Matt King in Northland's Kerikeri was complete with a shoutout from the crowd to "our King in the North".

She then welcomed frontbenchers onto two socially-distanced couches to chat policy, before Ms Collins delivered a 20-minute address.

Ms Collins whacked Ms Ardern's government for mistakes made with its mandatory isolation regime, its plans for higher taxes and a failure to deliver signature policies.

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

During the speech Ms Collins called it "bittersweet" to only have 80 socially-distanced fans in the crowd but afterwards she said she enjoyed it.

"I thought it was professional. It was slick. It was fabulous. It was modern and it was now," she said.

She downplayed the costings mistake by saying "nothing Grant Robertson says worries me".

Ms Collins returns to centre stage on Tuesday night in Auckland when she confronts Ms Ardern in the first debate of the campaign, screened on TVNZ.

Ms Ardern's Labour is a huge favourite to win a second term.

Australian Associated Press