The Prime Minister said decisions on the reopening of New Zealand's international border will be reviewed when they have more information on the Omicron variant in early January.
Ardern acknowledged enormous progress had been made in Northland but said they must be cautious.
The next full review of the traffic lights will be on January 17.
The South Island "is very well placed" to move to green from then, Ardern said, but they are trying to find a balance while moving into the new framework.
She said right now a lot of people in the South Island like the comfort of knowing they are surrounded by vaccinated people.
Ardern said areas will move into green, but not while the country is in a "transitional period".
Ardern said the decision to move Auckland to orange on December 30 was based on the transmission cycles, or blocks, of two weeks.
"If you count forward from the 3rd, you land on the 30th," she said.
Vaccine passes will be in use up and down the country, Ardern said, and people will be able to gather together because it is safe to do so.
Ardern announced the decisions after Cabinet met today.
Cases and hospitalisations are all running slightly under the levels predicted, Ardern said.
The country is about 47,000 vaccines away from hitting the 90 per cent target, the Prime Minister said.
In light of the Omicron situation, ministers will receive the latest advice in early January and check-in on its plan to open up the country's borders against the spread of the new variant, Ardern said.
Ardern said New Zealand was one of the countries that had done the least in response to Omicron so far, and she would have to consider evidence in the new year.
Ardern said countries that had removed vaccine passports such as Denmark have recently reinforced them in the wake of Omicron.
"We do see those who have moved away have then gone back to them," she said.
She said many countries don't have a framework that offered pathways for people without vaccine passports, like New Zealand has.
Rapid antigen tests will be available at 149 pharmacies around Auckland from Wednesday.
Results are available after 15 minutes, and Ardern urged people who were not vaccinated and wanted to travel to use it.
Ardern said the border around Auckland remains important despite the move to orange. She said part of the success of containing Delta has been the border.
It is not a onerous ask for the vast majority of Aucklanders, who are vaccinated, she said.
They just needed to carry proof of vaccination to move through the border.
Northland
Ardern said that Northland was 4000 shots shy of meeting a 90 per cent first-dose target.
She said the difference between Northland and other regions is those few thousand vaccinations. The decision to keep it in red is about being cautious.
Ardern said not every car travelling north of Auckland will be stopped by police, and the flow of traffic will be maintained.
She said police are operating the checkpoints and they will be taking into account traffic flow. Ardern said checkpoints can only be legally run by police, and police are ultimately the ones in charge.
But Ardern said operational decisions are made by police, not her.
Ardern said vaccination numbers were "significant" in deciding to keep Northland in red.
Booster shots
The vast bulk of New Zealanders who are eligible for boosters will come up to their opportunity for the booster around March and May.
New Zealand director-general of health Dr Ashley Bloomfield said it was reassuring that Pfizer is effective against Omicron, especially with the third dose.
By Christmas, about 450,000 people were due for a booster, Bloomfield said, and he strongly encouraged them to do so.
Bloomfield said winter is a problem with the spread of Omicron overseas.
Today's case numbers
Earlier today, the Ministry of Health reported 101 new Covid-19 cases and 61 people with the virus in hospital, four of who are in ICU.
Today's Covid cases are in Auckland (97), Bay of Plenty (1), Taranaki (1), Nelson-Marlborough (1) and Canterbury (1).
Ardern told RNZ this morning it was still early days of transitioning into the traffic light system and the Government wanted to ensure they saw the full impact of that was known before easing more.
"We will take an approach that will stand us in good stead. One thing we are mindful of is if you do move too soon you run the risk of escalating cases and you end up in higher restrictions. We want to go the distance."
Te Pūnaha Matatini Covid-19 modeller Professor Michael Plank has also cautioned that 10 days was still a bit too early to judge whether the move from lockdown to traffic lights had impacted on the outbreak.
"Over time if cases in Auckland remain at a manageable level or even continue to fall I think there would be a strong case for moving Auckland to orange.
"But I think a cautious approach is warranted because we are still yet to see what effect it is going to have."
He said it was also important to keep cases as low as possible ahead of Auckland's boundary re-opening and summer travel – to reduce the risk of Covid-positive people getting to low vaccinated regions.
Today's decisions come two days before Auckland's boundaries are due to re-open.
From Wednesday, people can move to and from the city if vaccinated, or if they have a negative Covid-19 test.
Rapid antigen testing will also be available from Wednesday at pharmacies for those wanting to test themselves before travelling for summer.
