New Zealand's Prime Minister Jacinda Ardern put the nation under strict lockdown on Tuesday after one new case of the coronavirus was reported in its largest city of Auckland, the country's first in six months.

All of New Zealand will be in lockdown for three days from Wednesday while Auckland and Coromandel, a coastal town that the infected person had also spent time in, will be in lockdown for seven days.

Imposing its toughest level 4 lockdown rules, schools, offices and all businesses will be shut down and only essential services will be operational.

"The best thing we can do to get out of this as quickly as we can is to go hard," Ardern told a news conference.

"We have made the decision on the basis that it is better to start high and go down levels rather than to go low, not contain the virus and see it move quickly," she said.

Ardern said authorities were assuming the new case was a Delta variant infection although this has not been confirmed. There may be other cases, she said.

The last reported community case of COVID-19 in New Zealand was in February.

New Zealand has followed a go-hard-and-early strategy that has helped it virtually eliminate COVID-19 domestically, allowing people to live without restrictions although its international borders remain largely closed.

The country has reported about 2,500 confirmed cases of coronavirus and 26 related deaths.

The New Zealand dollar tumbled on the announcement, falling 1.5% to $0.6926 after the lockdown was announced.

The news came a day before the Reserve Bank of New Zealand is expected to become the first central bank among developed countries to raise interest rates since the pandemic as the economy booms.