The current lockdown in Auckland will be in place for at least three days, with Ms Ardern saying that will give officials time to assess the situation and test tens of thousands of people in the coming days. This means that people will be asked to stay at home and bars and many other businesses will be closed.
New Zealand is now activating a resurgence plan.
New Zealand instituted a ban on travel to and from China, where COVID-19 appears to have originated, on 3 February, with all New Zealand residents returning from China after that date compelled to quarantine for 14 days.
"It was perhaps easy to feel New Zealand was out of the woods, my request is not to feel dispirited or disheartened", she said.
The original case has no history of overseas travel, so all close contacts and work colleagues across multiple workplaces have been tested and placed into isolation.
Director-General of Health Dr Ashley Bloomfield today repeated his thanks to Global Positioning System and other front-line services which continued to provide health services in the challenging circumstances we were all working under.
In June New Zealand saw its first two cases of COVID-19 in 24 days, with two women who had arrived in New Zealand from the United Kingdom testing positive after being granted a compassionate exemption from isolation to visit a dying relative.
Ardern said: 'We have not been able to trace the sources of these cases.
Earlier today the dissolution of Parliament was delayed after the re-emergence of Covid-19 in the community, with the Government seeking further advice around how the new cases may affect the September 19 election.
As a result of the infections, mobile phones nationwide received an emergency text that set off blaring alerts and said "if you are in Auckland STAY HOME. and SAVE LIVES".
New Zealand Prime Minister Jacinda Ardern speaks during a joint press conference held with Australian Prime Minister Scott Morrison at Admiralty House in Sydney, Australia, February 28, 2020.
Auckland was swiftly put under a three-day lockdown after four cases were confirmed in one family there.
Under New Zealand law, the government could delay the election for up to about two months.
130 people who work with the man who tested positive in a finance company are considered close contacts.
The World Health Organization website states there is now no confirmed case of COVID-19 transmitted through food or food packaging.
Police were setting up roadblocks around the city and supermarkets began rationing the sale of some staple products amid a rush to the shelves.
"We're expecting to see other cases", said Ashley Bloomfield, New Zealand's director-general of health.
"There's no need, they're an essential service", she said. We all had hoped not to find ourselves in this position again, but we had also prepared for it.
There are now 22 active cases of Covid-19 in the country, with the total number of cases being 1,220.