New Zealand Prime Minister Jacinda Ardern on Wednesday delayed a key step toward next month's general election, as the country was plunged back into lockdown after the discovery of the first COVID-19 cases in more than three months.

Ardern said she was suspending the dissolution of parliament, which was due to make way for an election scheduled to take place on Sept. 19, until Monday. No decision had yet been made on delaying the actual poll, she added.

"It's too early to make decisions but there is a bit of flexibility to move the election date if required," Ardern told a televised media conference, adding any date before Nov 21 can be chosen for the election.

New Zealand health officials are urgently tracing the source of four new cases of COVID-19, all within one family in Auckland, the country's largest city.

The cluster, the first cases reported in the country for 102 days, led Ardern to announce the reimposition of lockdown measures late on Tuesday.

Auckland shifted to level 3 restrictions at midday (2400 GMT) on Wednesday, requiring people to stay at home unless for essential trips. The rest of the country was placed back into slightly looser level 2 restrictions. The restrictions will initially remain in place until Friday.

"Stay kind, looking after one another," Ardern said, adding that the search for the source of the infection was ongoing. The family had not travelled overseas.

"We know that although we are primarily look for human to human transmission we do need to check whether or not it had something to do with freight," she said.

Director General of Health Ashley Bloomfield said the infected family was in strict isolation at home, but one woman had travelled to tourist spots in Rotorua while she was symptomatic.

Health officials were urgently tracing her activities and were prepared to test tens of thousands of people in the coming days, Bloomfield said.

Ardern said aged care facilities would close their doors to everyone but staff and essential deliveries from midday.