Cabinet has agreed to make the Pfizer vaccine available for 12- to 15-year-olds after Medsafe gave it provisional approval for the age bracket in June.
Medical regulators in Canada, Japan, Europe and the US have also granted a similar approval.
Ardern said cabinet did not make the decision lightly.
"Many of us are parents ourselves and take this duty of making decisions about other people's children extremely seriously but it is safe and it's the right thing to do."
From tomorrow, parents and caregivers will be able to book in 12- to 15-year-olds to receive their vaccination at the same time as their parent or caregiver.
Ardern said this was the practical thing to do.
"If you've already booked just bring them along please just make sure while you do that you work hard to keep up your physical distancing from others".
There are an estimated 265,000 people in the 12- to 15-year-old age.
The Prime Minister assured there is a sufficient supply of the Pfizer vaccine and expanding the age bracket to 12- to 15-year-olds will not affect vaccine availability for others.
"I want to reassure New Zealanders that our vaccine supply chain continues to operate well."
A phased approach to re-opening vaccination centres started today and with some less urgent appointments cancelled due to level four many centres now have extra capacity, Ardern said.
The government plan to use this added capacity to increase the vaccination rate of essential workers.
Priority will be given to public facing workers such as supermarket workers who may not have been eligible for vaccination due to their age bracket, Ardern said.
"We're working with employers of these essential workforces to identify the individuals and workers can expect to hear from your employer with extra details," she said.
"We're also working with the covid vaccination healthline on a process for smaller businesses and people who are self-employed to enable them to book if they too fit that criteria."
A record 195,000 vaccination bookings were made yesterday, Ardern said.
Vaccination rates in Auckland
More than a quarter of Aucklanders are fully vaccinated against Covid, with 44 percent having at least some level of protection.
The Ministry of Health says about 357,000 people - or 26 percent - have been given two doses.
A further 239,000 have had only their first dose.
The rates are slightly about the national average.
One dose of the Pfizer vaccine does offer some level of protection against the virus to most people, but two doses are much more effective.
