The lack of known links between new community cases of COVID-19 and travel or isolation facilities "means there are almost certainly other active cases present in Auckland", an expert says.

Prime Minister Jacinda Ardern and Director-General of Health Dr Ashley Bloomfield announced on Tuesday that four individuals from a single family in south Auckland had tested positive for COVID-19. They are the first cases in the community for more than 100 days.

In an attempt to go hard and go early, and stop the virus spreading, the Government announced Auckland will move to alert level 3 on Wednesday, while the rest of the country goes to alert level 2. 

Responding to Tuesday's bombshell announcement, Professor Shaun Hendy, who conducted virus modelling for the Government earlier this year, said the lack of known links to international travel or to isolation facilities "means there are almost certainly other active cases of COVID-19 present in Auckland at the moment".

He said it's therefore important for people to behave accordingly and stick to our bubbles. Alert level 3 calls for Kiwis to limit travel and not leave the house unnecessarily.

"Moving to level 3 also reduces the number of contacts most of us have, which will give our contact tracers a much easier job over the coming days. It would also be worth Aucklanders checking their apps, diaries, and bank accounts to make a list of places they have visited or people they have had contact with over the last fourteen days," he said.

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"If you can, you should also wear a mask when going to the supermarket. Remember that not everyone can wear a mask, so wear yours if you can to protect them."

The Government last week asked New Zealanders to stock up on masks in case of a new outbreak. 

Prof Hendy said there is a chance the virus has spread to other parts of the country and all Kiwis should take necessary precautions.

Professor Michael Plank from Te Pūnaha Matatini and the University of Canterbury also said the lack of a known link to the border "makes the situation more serious".

"This means there could be several links in the chain and a larger number of cases that we don’t yet know about. That’s why it’s important to take swift, decisive action and move the whole of Auckland to alert level 3 as soon as possible," he said.

Prof Plank said it was important the new lockdown applies to the entire super city as people from the infected households work in different parts of the region. He noted that Melbourne - currently struggling with a second wave - tried to lock down specific suburbs and failed to contain the virus.

The rest of the country in alert level 2 should also be very cautious. It’s quite possible a case we don’t yet know about has travelled outside of Auckland and it would be easy for the virus to spread. 

"So everyone needs to get back to social distancing, work from home if you can, wear a mask if you need to go out, and get tested if you feel sick."

According to Dr Bloomfield, the index case for the family has no overseas travel history and has been symptomatic for five days. Close and casual contacts are being traced and household contacts have received a rapid test.

Three of these have come back positive and three negative. 

"All close contacts of the four cases will remain in self-isolation for 14 days, regardless of their test result, and all casual contacts will remain in self-isolation until they have the results of their test," he said.

"In addition, we are working over the next few days to test all people that are working at our borders and everyone that works at a managed isolation facility. "

Dr Bloomfield said anyone offered a test should take one. Community testing centres will be operating with more staff and extra hours across Auckland as well as pop-up clinics.