New Zealand Prime Minister Jacinda Ardern vowed Sunday to "crack on" with delivering her vision following a landslide election victory, after a string of disasters frustrated reform in her first term.

Ardern won a historic outright majority in Saturday's poll -- the first since New Zealand adopted proportional voting in 1996 -- allowing her to implement policies without support from minor parties.

After facing criticism since winning office in 2017 for not delivering on key promises such as protecting the environment and reducing child poverty, Ardern said she now had a mandate for change.

The charismatic leader said the scale of the victory, Labour's biggest since 1946, meant more voters were backing her centre-left party and its reformist agenda.

"I think they were endorsing the work we've done and the plan we have to go forward, and there are some areas we do want to crack on with," Ardern said.

The 40-year-old, who has been hailed internationally as a standard-bearer for progressive politics, admitted the need to placate minor-party coalition partners "slowed down" reform in her first term.

She also dealt with New Zealand's worst terrorist attack, a deadly volcanic eruption and the country's deepest recession in 30 years.

In her victory speech late Saturday, Ardern flagged increased state housing, more renewable energy and other infrastructure investment.

She also spoke of more training programmes, job creation, protecting the environment and a determination to tackle issues such as climate change, poverty and inequality.

- 'Covid election' -

Opposition leader Judith Collins conceded voters had given Ardern free rein to implement change, but said that also meant the prime minister could no longer claim her policy failures were caused by reform-shy coalition partners.

"The government has got the mandate to do all the things that they've promised to do, so they can't blame anyone else for not delivering," Collins told reporters.

Campaigning during the vote centred on the government's successful coronavirus response, with Ardern dubbing it the "Covid election".

New Zealand has recorded only 25 coronavirus deaths in a population of five million, which Collins said boosted Ardern's standing in an electorate anxious about the pandemic.

However, Ardern received a pointed reminder that the threat remained potent when health authorities on Sunday announced a new case of community transmission in Auckland, two weeks after she had announced: "We beat the virus again."

Asked if she had enjoyed a lie-in after an exhausting campaign, Ardern replied: "No, I was dealing with Covid."

She said the case appeared to be contained and would not prompt a change to New Zealand's virus alert level, currently on its most relaxed setting, Level One.

Her Labour Party's 49.1 percent of the vote was its best election performance since 1946, while National's 26.8 was the second-worst since the party was founded in 1936.

Collins said she would remain as National leader but commission an independent review into how the party's vote slumped almost 18 points from the last election in 2017.