The opposition National Party denounced the announcement, saying the new strategy would unleash a summer of chaos.
The Green Party said the Government was opening the border before vulnerable communities were ready.
"Elimination has protected thousands of lives in Aotearoa and maintaining elimination is still possible outside of Auckland for now," Green Co-leader Marama Davidson said.
But Ardern projected 90 per cent of all New Zealanders would be fully vaccinated by mid-December.
"We are in a new phase in our fight against Covid-19," Ardern said shortly after 1pm today.
To minimise the risk of Covid-19 spreading, travellers from Auckland will have to be either fully vaccinated or have a negative test within 72 hours of departure.
The vaccine and test rule will be in place from December 15 to January 17 next year.
"Aucklanders have faced restrictions for an extended period of time to keep the rest of New Zealand safe," Ardern said today.
"But with increased rates of vaccination it's time to open up the ability to travel again."
The PM said the December 15 date would provide certainty.
"Aucklanders can now book summer travel and accommodation with confidence and businesses inside Auckland and around the rest of the country can plan for summer travellers."
Although Aucklanders could leave, the changes mean people from across New Zealand can also visit the country's biggest city from mid-December.
Police will carry out random spot checks and, Ardern said, will work with iwi on the northern boundary.
Ardern said the Government had carefully considered what summer protocols might need to apply to the South Island.
She said Air New Zealand had vaccine or testing requirements. The Interislander ferry had potential to carry out similar steps, and Ardern said the combination of plane and ferry measures could slow any Covid-19 spread to the south.
In regions with low vaccination rates, some tighter restrictions may remain for longer than in places with high vaccination rates.
Traffic lights for everyone
Another significant announcement could be made in 12 days about the traffic light system.
Cabinet will confirm on November 29 its decision to move Auckland into the new traffic light system.
Auckland will initially move into red, the highest level. After days of speculation, Ardern confirmed all regions will move into the new traffic light framework with Auckland.
The three-level traffic light model replaces the country's four-step alert level system, which was introduced more than 18 months ago.
Old model booted
Ardern said elimination and the four alert levels were never intended as a "forever strategy".
Today, the PM said high vaccination rates offered a chance to move into the next phase of the pandemic response.
Places with lower vaccination rates will move into the new system at red, which Ardern said had greater protections than the current alert level 2.
She said the steps announced today balanced the need to give Aucklanders mobility, while reassuring the rest of New Zealand steps were in place to slow the spread of Covid-19.
"Eighty-two per cent of New Zealanders are now fully vaccinated compared to just 23 per cent when Delta arrived three months ago, so we are in a much safer position to ease boundary restrictions."
Ardern said inevitably, some people would query the timing of the new system's announcement.
She said the old pandemic responses had served their purpose.
"We've had a plan and it's carried us through."
Restaurants confused
The Restaurant Association said today's announcement was not helpful for the beleaguered hospitality sector.
"Our industry has once again been left with more questions than answers," Restaurant Association chief executive Marisa Bidois said.
She said there was rampant confusion about what role vaccine certificates or passports might have when hospitality opens up.
Businesses had little or no information on how passports would be used, but customers had many questions, Bidois said.
"There is also no information on whether businesses are required to have a person on the door admitting people or turning them away."
And Bidois said it was still unclear if vaccine passports would be needed for takeaways, or for outdoor dining.
Ardern said there was a simple answer for many queries: "If you want to be open and you want to have as many customers as possible, you will need to run vaccine certificates."
She said where patrons needed certificates, it was only fair for staff to be vaccinated too.
