A quiet area of Christchurch Airport is pictured during the coronavirus lockdown in Christchurch. AFP

New Zealand reported a breakthrough on Thursday in tracing the source of a Covid-19 outbreak that plunged the nation into lockdown, as questions grew about the government's response to the pandemic given the slowest vaccination rate among developed countries and the economic pressures of prolonged isolation.

Prime Minister Jacinda Ardern saying it should help "stamp out" the virus.

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Although, health officials have been trying to determine how an Auckland man contracted the coronavirus this week, ending a six-month run of no community cases in New Zealand.

Eleven new cases were reported on Thursday, taking the total to 21 in the latest outbreak that ended the country's six-month, virus-free run.

Prime Minister Jacinda Ardern

Tests showed the man had a version of the Delta strain found in Australia, and Ardern said investigations narrowed down the origin to a person who arrived from Sydney on August 7.

She said the traveller had been in quarantine and hospital since touching down, indicating the virus had not been in the community as long as initially feared.

"We believe we have uncovered the piece of the puzzle we were looking for," Ardern told reporters.

"This is a significant development. It means now we can be fairly certain how and when the virus entered the country," Ardern said at a news conference.

"And the period in which cases were in the community were relatively short."

She said finding the outbreak's source also increased the "ability to circle the virus, lock it down and stamp it out".

Officials said it was still unclear how the virus spread into the community and 1,000 close contacts of positive cases were being assessed.

A decision is due Friday on whether the three-day lockdown will be extended or end by Saturday.