The election had been scheduled for September 19 but will now be held on October 17, Ms Ardern announced on Monday, as the number of confirmed cases rose to 49.
Opposition parties had called for the delay because they said sticking with the original was preventing them campaigning with the country’s largest city Auckland on lockdown for two weeks.
Under New Zealand law, Ms Ardern had the option of delaying the election for up to about two months.
Ms Ardern’s Labour party is expected to win a second term with it being significantly ahead in the polls, but it has faced mounting criticism and anger over the re-emergence of the disease after 102 days of no new cases in the community.
Ms Ardern said: “Ultimately I want to ensure we have a well-run election that gives all voters the best chance to receive all the information they need about parties and candidates, and delivers certainty for the future.”
The September date had been relatively early in New Zealand's three-year election cycle, with Saturdays in November often being polling day.
The only known cases when New Zealand was “Covid-free” had been residents or citizens returning from overseas who had quarantined at the border.
Officials have still been unable to figure out the origins of the outbreak a week ago and the government has faced questions over why large numbers of staff at the border have not been tested for the disease.
Authorities say they believe all 49 cases are connected, raising the hope that the virus is not spreading beyond the cluster.
Associated Press contributed to this article