She said an extra four days in a national lockdown would give health officials more COVID-19 test results, data and a "level of reassurance".
The national lockdown, excluding Auckland, will be assessed on Friday, Ardern said. Auckland's alert level settings will be reviewed next Monday.
It comes after New Zealand's COVID-19 outbreak grew by 35 cases on Monday, bringing the cluster's total to 107.
The outbreak was sparked by a 58-year-old man testing positive for the highly infectious Delta strain in the Auckland suburb of Devonport last Tuesday. The case has since been linked to a returnee from Sydney in managed isolation, but how the infection was passed on remains under investigation.
Ardern said on Monday COVID-19 cases may still continue to grow before declining.
She said the Delta variant "had a head start" on New Zealand and there were significanly more contacts and locations of interest than previous outbreaks.
"We have some way to go yet," Ardern told reporters.
She said the Government and Ministry of Health remained confident of containing the outbreak.