AUCKLAND  — New Zealand locked down nursing homes nationwide on Wednesday after a 102-day streak without the coronavirus ended, as Prime Minister Jacinda Ardern said the outbreak could force her to postpone next month's general election.

Ardern said authorities were scrambling to trace anyone who had been in contact with four Auckland residents who tested positive on Tuesday, ending the dream run in which the virus had been contained at New Zealand's borders.

A three-day stay-at-home order for Auckland, New Zealand's biggest city with a population of 1.5 million, was announced on Tuesday night and went into force at lunchtime on Wednesday.

Police in facemasks manned roadblocks on major roads in Auckland to enforce the new measures.

Ardern said health officials were also locking down aged care homes across the country because they could act as transmission hotspots.

"I realise how incredibly difficult this will be for those who have loved ones in these facilities, but it's the strongest way we can protect and look after them," she said.

There was panic buying at supermarkets across New Zealand and huge queues at coronavirus testing stations as Kiwis came to terms with the re-emergence of a virus many thought had been defeated.

New Zealand had been held up by the World Health Organisation as an example of how to contain the disease after recording only 22 deaths in a population of five million and preventing community transmission for more than three months.

Ardern said the return of coronavirus was "unsettling" but all efforts were being made to retrace the steps of the Auckland family of four who contracted it from an unknown source.

She said the September 19 election may be impacted if the outbreak could not be contained.

"We're seeking advice from the Electoral Commission, just so that we make sure have all options open to us," she said.

"No decisions yet, as you can imagine, have been made."

Ardern's centre-left Labour Party has been riding high in opinion polls and is expected win a second term.

The conservative National Party was open to the idea of a delay if conditions meant it was justified.

"It's going to be very difficult to have an election in mid-September when we are now mid-August. It is very little time," National leader Judith Collins told TV3.  — AFP