An epidemiologist has outlined the "best case scenario" for the country after the Prime Minister announced four cases of community transmission were plunging Auckland back into lockdown.

Otago University epidemiologist Professor Michael Baker hoped mass testing would reveal a link to an overseas traveller and all possible chains of transmission could be quickly isolated, with very few positive cases.

"And the testing around those cases doesn't reveal an extensive outbreak. That's the best possible outcome."

In this scenario, Baker was hesitant to say how long Auckland should stay at alert level 3 to be confident that all potential chains of transmission were snuffed out.

The incubation period of the virus has previously led the Government to flag lockdown timelines in two-week periods.

But Prime Minister Jacinda Ardern wouldn't say if Aucklanders should expect to be in level 3 for at least two weeks, saying the testing in the next three days would reveal more to inform the next step.

Baker said it was "more likely" that the source of infection will be harder to find.

He said it was possible that there were already several chains of transmission, given that some of the Covid-positive family members had strong symptoms for five days before being tested and had been in at least two workplaces.

Chains could also be spreading around the country, given that New Zealanders had no travel restrictions.

Baker said more will be known in the coming days, and he praised the Government for what he said was a "quick and rigorous" response.

• Helen Clark prescient warning on country's Covid-19 response
• Nationwide lockdown 'almost certain', CovidCard decision needed now, Trade Me founder says
• Expert's warning of 'almost certainly other cases' after Auckland community transmission
• Everything we know about the Auckland cases of community transmission

"The problem in Victoria was the response was too slow and not decisive enough. We learned back in March that a rigorous response makes all the difference.

"We had the most stringent lockdown anywhere, and it really worked - dramatically. Now we have better testing and contact-tracing capacity than we did then, plus we've embraced the use of masks.

"Obviously it's a shock after this length of time of no community transmission to have an outbreak, but that's in the nature of this global pandemic."

Testing rates had dropped since the end of June, and Baker said if they had remained higher it is "possible" that these cases, or chains of transmission linked to them, could have been detected sooner.

But that was impossible to know, he added.

University of Auckland Professor Shaun Hendy said there were "almost certainly other active cases of Covid-19 present in Auckland at the moment".

"However, there is also a chance the disease will have spread to other parts of the country. Because of this everyone in New Zealand should take the necessary precautions, including wearing masks if you can, especially in close indoor settings, hand-washing, and getting tested if you have any of the symptoms.

"Stay safe, and be kind."