WELLINGTON (REUTERS) - New Zealand announced on Tuesday it was shutting down its largest city, Auckland, after four new cases of Covid-19 were discovered in the city, the first evidence of domestic transmission after being coronavirus-free for 102 days.

Prime Minister Jacinda Ardern said Auckland would move to level 3 restriction from noon on Wednesday as a "precautionary approach", which would mean people should stay away from work and school, and gatherings or more than 10 people are again restricted.

The restriction would be applied for three days until Friday.

Director General of Health Ashley Bloomfield said the four confirmed cases were within one family. One person is in their 50s. They had no history of international travel. Family members have been tested and contact tracing is under way.

"This is something we have prepared for," Ms Ardern said in a surprise news conference, adding that the increased caution was as the source of the virus was unknown.

"We have had a 102 days and it was easy to feel New Zealand was out of the woods. No country has gone as far as we did without having a resurgence. And because we were the only ones, we had to plan. And we have planned."

Travel into Auckland, on the North Island, would be restricted unless you live there, she said.

Ms Ardern also said the rest of New Zealand was entering alert level 2 from midday Wednesday for three days. This would mean social distancing measures would be applied again. She urged people not to rush to supermarkets to stack up.

New Zealand marked 100 days without a domestic transmission of the coronavirus on Sunday, but warned against complacency as countries like Vietnam and Australia which once had the virus under control now battle a resurgence in infections.

Separately, a New Zealand retirement village in Christchurch, in the South Island, has gone into lockdown after residents displayed symptoms of respiratory illness, the New Zealand Herald reported on Tuesday.