The election had been scheduled to be held on September 19.
Pressure had been mounting on Ardern to postpone the vote amid the resurgence of COVID-19 infections in its biggest city Auckland, after the country had been free of coronavirus cases for 102 days.
New Zealand Deputy Prime Minister Winston Peters called on Sunday (August 16) for a delay to the election.
The main opposition National Party also wanted a delay, after they were forced to cancel campaign events due to restrictions on movement and crowds due to the pandemic. It has accused Ardern of using the crisis to shore up support.
Ardern said she had advised the Governor-General of the new election date, and added she did not intend to change it again.
New Zealand law requires the election to be held before November 21.