WELLINGTON: New Zealand's coronavirus cases jumped on Thursday, 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.

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 said the virus has not been in the community for long as authorities had linked its origin to a returnee from Sydney on Aug 7.

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

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

New Zealanders had been living without curbs until Ardern ordered a snap 3-day nationwide lockdown on Tuesday after a case was found in the largest city Auckland, the first in the country since February.

Ardern, who shut the country's borders in March 2020, had announced plans for a gradual reopening this month following pressure from businesses and public sectors facing worker shortages that policymakers fear will fuel inflation.

The new cases may delay those plans and are causing concern in the nation, which has struggled to get its population vaccinated.

Only about 23% of its 5 million people have been fully vaccinated, the lowest rate among the 38 members of the Organisation for Economic Co-operation and Development (OECD).

"It's no longer clear Jacinda Ardern's strategy is the right one," read one opinion piece in the New Zealand Herald.

Opposition National Party leader Judith Collins labelled the vaccination rollout as a failure.

Experts also said vaccinating everyone was the way to return to some normalcy.

"The virus is out there. We cannot go on thinking we will keep it out forever," said immunologist Graham Le Gros, Director of Malaghan Institute of Medical Research and Programme Director at Vaccine Alliance Aotearoa New Zealand.

"I think what is critical is that as a nation we now seriously focus on getting as many people vaccinated as possible, it is the only way our country can return to normality for the sake of our health and people’s livelihoods."

Apart from some anti-lockdown protests, New Zealanders mostly followed rules and stayed at home. The government warned not doing so will result in chaos, as seen in neighbouring Australia.

"We have seen the dire consequences of taking too long to act in other countries, not least our neighbours," Ardern said.