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The rest of New Zealand will be at level 2 over the same time period, Jacinda Ardern announced Sunday afternoon.
She said the move was "the right thing to do" and was a "cautious approach".
"I'm asking New Zealanders to continue to be strong and to be kind."
She said we all have this feeling of "not again… but we are going to be okay".
Leader of the Opposition Judith Collins and Act leader David Seymour criticised the Government for putting Auckland into another lockdown.
Collins, who is returning to Wellington while her Auckland MPs stay put in Tāmaki Makaurau, said the latest lockdown move will be a "devastating blow" for New Zealanders.
"This will be difficult news for all New Zealanders.
"I feel for the people who have already sacrificed their livelihoods, their jobs and their businesses to keep Covid-19 out."
Collins urged people to stay vigilant in the fight against Covid-19 and follow the advice of health officials.
Finding the source of the outbreak and vaccinating border workers was a matter of urgency, she said.
She said if this proves to be another border failure that is unacceptable.
New Zealand is preparing for the arrival of the first batch of Covid-19 vaccines this week. Ardern announced on Friday that frontline Covid-19 workers will get the vaccines first.
After those roughly 12,000 workers are vaccinated, their close family will be offered the jab.
Collins said National has been calling for an accelerated rollout of vaccinations for border workers and frontline staff since January.
"We were acutely aware of the economic and social damage a third lockdown could cause. Avoiding this very scenario at all costs should have been the Government's top priority."
Collins attended the 22nd Big Gay Out in Auckland, after Ardern had to ditch her appearance at the event to race to Wellington in the wake of three new community cases.
Seymour said it "was only a matter of time before an outbreak forced another lockdown".
He called New Zealand a "sitting duck, saved by luck, citizen cooperation and geographic isolation".
"The Government did not take the opportunity to strengthen our defences with better testing and contact tracing, as the Prime Minister claimed. Instead it pinned its hopes on a vaccine while inviting the Wiggles through MIQ.
"We must hope that the Government's testing and tracing can isolate this outbreak before a damaging extended lockdown harms livelihoods."
"Last month the Government tried to say New Zealand's position was so good we could afford to wait for a vaccine. Act said at the time that we are only an outbreak away from wishing we had one.
Ardern took to Facebook Sunday evening to answer questions from Kiwis on the alert level move.
"Basically, what we want everybody to do is stay home," she said.
"We are asking people from Auckland not to leave."
But there are some exceptions such as those that live on or near the Auckland border needing to travel into the region for legitimate purposes.
She said anyone who got an exemption last time can anticipate getting one this time.