The vote was due to take place on 19 September but will now be held on 17 October instead.
Ms Ardern said on Monday that the new date would allow parties "to plan around the range of circumstances we will be campaigning under".
Earlier this week, the country's largest city went back into lockdown.
"This decision gives all parties time over the next nine weeks to campaign and the Electoral Commission enough time to ensure an election can go ahead," Ms Ardern said, adding that she had "absolutely no intention" of allowing any further delays to the vote.
The opposition National Party has argued the election should be delayed as restrictions on campaigning mean Ms Ardern had an unfair advantage.