All New Zealand will be in the toughest, level-four lockdown for at least three days from Wednesday. Auckland and Coromandel, a coastal town where the infected person also spent time, will be in lockdown for seven days.
"The best thing we can do to get out of this as quickly as we can is to go hard," Ardern told a news conference.
"We have made the decision on the basis that it is better to start high and go down levels rather than to go low, not contain the virus and see it move quickly."
Schools, offices and businesses will close and only essential services will be operational.
Shoppers crowded supermarkets as news of the new case and the imminent lockdown spread.
Ardern said authorities were assuming the patient, a 58-year-old man from Auckland who was not identified, had contracted the highly contagious Delta variant of COVID-19 although genome sequencing was yet to be completed.
There may also be other cases, she said.
The Auckland Regional Public Health unit is interviewing the patient for contact-tracing purposes and COVID-19 vaccinations are to be suspended for at least 48 hours.
'Lifting our game'
The last reported community case of Covid-19 in New Zealand was in February. Its citizens have been living without restrictions although international borders have largely remained closed.
The country has overall reported about 2,500 confirmed coronavirus cases and 26 related deaths.
Ardern said that under the level-four lockdown people must stay at home, leaving only for essential services. Protective face masks should be worn at all times when leaving home.
"Beating Delta means lifting our game," Ardern said.
"I ask New Zealanders to please follow the rules to the letter. We know from evidence overseas that the Delta variant can spread just by walking past someone."
The New Zealand dollar tumbled on the announcement, falling 1.5 per cent to $0.6926 after the lockdown was announced.
The Reserve Bank of New Zealand is widely expected to raise interest rates on Wednesday. But one bank, Westpac, said it now expected the central bank to keep the official cash rate (OCR) unchanged.