New Zealand Prime Minister Jacinda Ardern extended a lockdown in the country's biggest city on Friday in response to the first national coronavirus outbreak in months, sticking with a "go early, go hard" approach she said has proven effective.

Ardern said genomic testing has shown the latest outbreak is a different strain to the original outbreak in New Zealand earlier in the year, suggesting it was new to the country.

The New Zealand leader said lockdown measures in Auckland, home to about 1.7 million people, and social distancing measures across the country that were imposed on Wednesday would remain in place for another 12 days.