Prime Minister Jacinda Ardern has announced Auckland will go into lockdown and the rest of the country will go into level 2.
It comes after four cases of community transmission from the same family were confirmed - after 102 days without any.
Ardern and director general of health Ashley Bloomfield announced the news tonight and an alert was sent to New Zealanders' phones.
As of noon tomorrow, Auckland will move to level 3 for three days until midnight on Friday.
The rest of the country will move to alert level 2 at midday tomorrow - until midnight on Friday.
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For Auckland, that means working from home unless you are an essential service worker.
The area covered by level 3 is the Auckland Super City - from Wellsford in the north to Pukekohe in the south.
Schools will close unless they had students whose parents were essential workers. Public facilities, bars, restaurants and businesses must close tomorrow.
"We're asking people in Auckland to stay home to stop the spread," Ardern said.
"Act as if you have Covid, and as though people around you have Covid."
Essential services such as pharmacies and supermarkets will remain open, but Ardern said there was no reason to panic-buy.
"Please do not rush to the supermarket tonight ... there is no reason to rush out and back for any purchases this evening.
"You'll even be able to get a takeaway coffee."
Aucklanders should use a mask if they leave the house to access essential services. The rest of the country should wear a mask in situations where they can't physically distance, such as on public transport.
People who are in Auckland who need to go home to another region can do so but they should keep an eye on their health. There will be more advice tomorrow for these people and those who have recently travelled from Auckland.
Ardern said Aucklanders should keep an eye out for their neighbours.
"We have come too far to go backwards. Be strong and be kind.
"We know what to do because we have successfully done this before."
She acknowledged Kiwis might now have a sinking feeling but said they should know we had gone the longest of any country and there was a plan being actioned.
"My request is not to be dispirited or disheartened."
Ardern said imposing level 3 on all of Auckland, rather than a smaller part of Auckland, was about taking a precautionary approach.
Bloomfield advised aged residential care facilities to stop all visits, given how vulnerable older people are to Covid-19.
New Zealand went into level 4 lockdown on March 25, and moved to level 3 on April on April 27. We moved into level 2 on May 13 and level 1 on June 9.
The new cases
The four confirmed cases are in one family acquired from an unknown source. There was no link to overseas or managed isolation.
An Auckland man in his 50s was tested yesterday after having symptoms. He had no history of overseas travel and was tested a second time today. Both tests were positive.
Six other people were in his family, and three of them have tested positive. The other three tested negative.
They remain in their home for now but Ardern said moving them to a quarantine facility was being considered.
Their close contacts have been isolated for 14 days regardless of their test results. Casual contacts are also being isolated and cannot leave until they test negative.
Bloomfield said he was expecting to find other cases because they didn't know the source of the infection.
He declined to give the ages of the confirmed cases, but one child had tested positive, a pre-schooler who did not attend an ECE facility.
The man had strong symptoms including a fever and a cough, and the man's partner also had strong symptoms, Bloomfield said.
There were five days from symptom onset until they were tested, he said, which was why close contacts were being followed up.
He wouldn't say what suburb they live in in South Auckland, and the two workplaces were "beyond South Auckland".
Ardern said more information would be shared, but privacy issues needed to be considered.
Bloomfield said he was advised just before 3pm today and Ardern said she was told about 4pm.
More questions needed to be answered, Ardern said, including tracing the case to its source.
"There is no immediate link we have found as yet to a managed isolation facility, and there is no connection to a high risk person, such as those who work at the border."
Without knowing that, a precautionary approach was needed, she said, "as disruptive as it is".
"One of the most important lessons we've learned from overseas is to go hard and go early to stamp out flare ups.
"We will be asking Aucklanders to take swift action with us."
Bloomfield said he didn't have the detail about the two workplaces in question, but immediate action had been taken. One of the workplaces had been shut down already.
Neither workplace was customer-facing, he said.
Workers at the border are also being tested.
"I know this information will be very difficult to receive," Ardern said of the new cases.
"As a team we have also been here before. We know if we have a plan and we stick to it we can work through very difficult situations."
Bloomfield was not concerned that the cases might be linked to the person who tested positive after travelling from New Zealand to South Korea. Ardern added that the close contacts around that person all tested negative.
Ardern said if there were undetected cases in the community, there would likely have been hospital admissions, and their absence suggested the spread was less likely to be wider.
Moving back into lockdown was a potential scenario, Ardern said, but there will be daily updates to inform the public on the progress of the testing of close and casual contacts.
"These are very significant decisions. Moving an entire city, our biggest city, to level 3 is not a decision we take lightly. We need to make sure we have as much information as possible."
Auckland Mayor calls for calm
Auckland Mayor Phil Goff is tonight calling for calm and patience as the city goes into another lockdown.
"I understand that people are probably feeling a little bit scared, a little angry and a little confused right now. None of us wanted to go back into a lockdown, but we always knew this was a very real possibility," said Mayor Goff.
"I am urging Aucklanders to come together like we did last time to stamp out community transmission. Please remain calm, please do not panic buy and please follow the lockdown rules.
"Please follow Ministry of Health advice around good hygiene practices and social distancing. If you have symptoms, please make sure you get tested and avoid contact with others.
"We beat community transmission once and we can do again, but that requires all of us to work as a team.
"Like last time, more details will be forthcoming over the next little while, so please keep updated through news channel and official government websites.
A 'wake-up call' - Bloomfield
Bloomfield said additional testing will be available around the country and he expected an increase in demand. People with any symptoms should get tested, he said, whether they are in Auckland or not.
More staff will be placed at testing centres in Auckland, Waitemata and Counties Manukau.
There are testing sites at Northcote, Henderson, Grafton and Wiri. There will be more pop-up testing sites.
He said community transmission was inevitable as it was a tricky virus, and it was important now to "stamp it out".
"There are things every New Zealander needs to do."
Those included hand hygiene, coughing into your elbow, staying a home if sick, and seeking advice from doctors or Healthline if you have symptoms.
He urged people to use the Covid tracer app.
"Finally our contact tracing team will of course be in contact with anyone who may be identified as a casual or close contact."
If contacted, please return the call, he said.
The cases were a "wake-up call" against complacency.
"We have done this before and we can do it again."
Bloomfield said it was more relevant than ever to get tested if offered a test.
"Clearly there is a failure"
National leader Judith Collins told the Herald that she is calling off the rest of her campaign stops tomorrow and returning to Wellington.
She is in Auckland at the moment, but is "packing her bags" to get back to Wellington before level 3 starts tomorrow.
It's there she will base herself while she figures out what is happening in Auckland.
She said Ardern rang her tonight at 8:45pm to talk about the decision and the latest developments.
"I am, like I'm sure the rest of the country, extremely disappointed that this [Covid-19] has been allowed in through our borders.
"I'm pinning it straight on the borders – clearly it's come in through there; clearly it's not organic to New Zealand.
"We have had essentially a lockdown of our borders since March, and the only people allowed in have apparently been in quarantine – clearly there is a failure."
NZ First leader Winston Peters has temporarily suspended his party's campaign.
Ardern said she'd spoken to National leader Judith Collins about 8.30pm to advise her about the information about the case and the proposed course of action.
Ardern said her focus was on the immediate cases and hasn't given any thought to postponing the election yet.
Earlier today, Bloomfield said there was one new Covid-19 in managed isolation - a man in his 20s who arrived in New Zealand on July 30 from Melbourne.
He said the man tested negative on day 3 but went on to test positive on day 12.
The man had been in managed isolation at the Grand Millennium and has now been transferred to the Auckland quarantine facility.
There were 22 active cases all being treated in managed isolation facilities after testing detected the illness in returning Kiwis at the border.
It brought the total number of those who have had Covid-19 to 1220.
Bloomfield said work was already under way on planning a Covid-19 immunisation campaign, although a vaccine was yet to be developed.
"We are not sitting idle, we have work under way to look at how we could deliver that vaccine."
Earlier today two Christchurch rest homes went into partial lockdown after residents displayed flu-like symptoms.
Brookhaven Retirement Village in Woolston has reportedly tested eight of its residents and put them in isolation.
And the Village Palms Retirement Community in Shirley had advised family members in a letter that it was in lockdown after residents began displaying symptoms of a respiratory illness. Eight residents were displaying flu-like symptoms.
The Government ramped up its messaging last week despite the nation approached 100 days without community transmission.
Bloomfield confirmed masks could be mandatory in some situations if the country moved up alert levels. And there were also warnings about the likelihood of a "second wave" of infection.
"We all know, we can't afford to be complacent," Bloomfield said in a statement.
"We have seen overseas how quickly the virus can re-emerge and spread in places where it was previously under control, and we need to be prepared to quickly stamp out any future cases in New Zealand. Every person in the team of five million has a role to play in this."
National's deputy leader Gerry Brownlee said on Sunday the change in messaging was "very puzzling" and he called on the Government "come clean" about what it knew.
"People have made a big effort on this and they expect to get all the relative freedom," he said.
"None of us are complacent about it, I believe, it is something that is going to be with the world from this point on."
Moving into alert level 3 at midday tomorrow for three days.
Under Alert Level 3, you are encouraged work from home if you can.
Travel and self-isolation:
If you are in Auckland and do not live in Auckland, the Ministry of Health suggests you go home. Practise good hygiene and be conscious of your health. It is recommend bubbles are kept small.
Businesses are able to open, but should not physically interact with customers. Essential services including healthcare, justice services and businesses providing necessities are able to open. Bars and restaurants should close, but takeaways are allowed.
Schools in Auckland will have limited capacity. Where possible it is encouraged students learn from home.
When you're out and about:
Maintain physical distancing of 2 metres outside your home, including on public transport. It is highly recommended that you wear a mask if you are out and about.
Public transport can continue to operate with strict health and safety requirements. You should maintain physical distancing and wearing a mask.
Public venues should close. This includes libraries, museums, cinemas, food courts, gyms, pools, playgrounds and markets.
Gatherings of up to 10 people can continue, but only for wedding services, funerals and tangihanga. Physical distancing and public health measures should be maintained.
People at high risk of severe illness such as older people and those with existing medical conditions are encouraged to stay at home where possible, and take additional precautions when leaving home.
Rest of the country
Moving into alert level 2 at midday tomorrow for three days.
You can still continue to go to work and school, with physical distancing.
Wear masks if you can in public.
No more than 100 people at gatherings, including weddings, birthdays, funerals and tangihanga.
Businesses can open to the public if they are following public health guidance, which include physical distancing and record keeping.
People at higher risk of severe illness from Covid-19, for example those with underlying medical conditions and old people are encouraged to take additional precautions when leaving home.
Practice good hygiene - stay home if sick.