WELLINGTON, New Zealand—New Zealand placed its largest city, Auckland, into lockdown for seven days following the first case of Covid-19 in the community since February.

Prime Minister Jacinda Ardern said officials are assuming the new Covid-19 case is the more contagious Delta variant of the virus, given its global spread. Genetic sequencing that would confirm or disprove that assumption will be available Wednesday.

“Going hard and early has worked for us before,” Ms. Ardern told a press conference. “While we know that Delta is a more dangerous enemy to combat, the same actions that overcame the virus last year can be applied to beat it again.”

As the Delta variant sweeps the globe, scientists are learning more about why new versions of the coronavirus spread faster, and what this could mean for vaccine efforts. The spike protein, which gives the virus its unmistakable shape, may hold the key. Illustration: Nick Collingwood/WSJ The Wall Street Journal Interactive Edition

The seven-day lockdown for Auckland will close schools and most businesses and also be imposed on another region, Coromandel, that the infected person, a 58-year-old Auckland man, traveled to. All other parts of the country will be required to lock down for three days.

New Zealand has had just over 2,500 confirmed cases of Covid-19 and few deaths, according to data from its health ministry. The country has been helped by a strict initial lockdown last year and its remote island geography. Its borders have been closed to virtually all travelers, except those such as citizens and long-term residents, since March 2020.

The country has a policy of eliminating rather than slowing the spread of the virus, but health experts have warned that it is vulnerable to an outbreak of the highly contagious Delta variant given the domestic population’s low level of natural immunity.

New Zealand has fully vaccinated just 19% of its population, according to Our World in Data, lagging behind other developed countries such as the U.S. and U.K., where nearly half are vaccinated. Still, the government wants all New Zealanders over age 16 to be able to book a vaccine beginning Sept. 1.

Ms. Ardern, when announcing the Auckland lockdown, said all recent infections among overseas travelers in quarantine have been the Delta variant.

Neighboring countries, including Australia and Fiji, have struggled to contain outbreaks of Delta despite previous success in suppressing the spread of the virus. Australia’s two largest cities, Sydney and Melbourne, remain locked down as authorities battle Covid-19 outbreaks.

Write to Stephen Wright at stephen.wright@wsj.com