Australia has also currently retained its seven-day isolation period.
Ardern said the advice of experts and officials looked at emerging evidence around infectiousness and periods of infectiousness, as they related to variants.
They also factored in current case numbers.
"But the most important thing that we can be doing right now is making sure that people who have Covid are isolating with their household," Ardern said.
"That is a significant factor in reducing case numbers.
"You'd want to be careful, particularly in a winter period of making sure that you're not contributing to extra growth."
National Party Covid-19 response spokesman Chris Bishop said his party still supported reducing mandatory isolation periods from seven to five days, as it had since February, unless people were sick and still symptomatic.
He said that was in line with official guidance in the United States and other countries.
Bishop said it was more important to have a more clear testing-to-work regime, which allows household contacts deemed critical workers to continue working provided they return a negative test.
"Our view is that in this economy, all workers are critical. It shouldn't be based on an arbitrary definition of what a critical worker is.
"If you are a household contact and you don't have Covid, and you can prove that with a daily rapid antigen test, you should be able to go to work. And we've had that policy for three or four months now. We think it's time we introduced it."
On Monday there were 6910 new community cases of Covid-19 reported. The seven-day average of 8517 cases is down on the most-recent peak of 10,414 nearly two weeks ago.
There were 16 Covid-related deaths reported.
There are 836 people in hospital with the virus, including 27 in intensive care. The average age of people with Covid-19 in hospital is 65.
