The election had been scheduled for September 19 but will now be held on October 17, Ms Ardern announced on Monday.
Under New Zealand law, Ms Ardern had the option of delaying the election for up to about two months.
Opposition parties had been requesting a delay after a virus outbreak in Auckland last week prompted the government to put the city into a two-week lockdown and halted election campaigning.
“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,” said Ms Ardern, whose Labour Party is expected to win a second term.
Before the latest outbreak, New Zealand had gone 102 days without any known community transmission of the virus, and life had returned to normal for most people, who were going to restaurants, sports stadiums and schools without fear of being infected.
The only known cases during that time were returning travellers who were quarantined at the border.
Officials believe the virus was reintroduced to New Zealand from abroad but have not determined how.
The Auckland outbreak has grown to 49 infections, with authorities saying they believe all cases are connected, raising hope the virus is not spreading beyond the cluster.
