Coronavirus: 'Fifty-fifty' chance Auckland will have eliminated COVID-19 by end of September - Jacinda Ardern

Even Auckland's extended stay at alert level 2 will only give the city a 50-50 chance of having eliminated community transmission of COVID-19, Prime Minister Jacinda Ardern says.

The city will move from the makeshift alert level 2.5 to level 2 at 11:59pm on Wednesday, loosening restrictions on gathering sizes. The rest of the country moved from level 2 to level 1 overnight, removing all gathering restrictions

The number of new cases reported each day over the past week could be counted on one hand, and a few days have had zero. 

But Auckland will remain at alert level 2 for at least another two weeks - the usual maximum incubation period for the deadly virus - to decrease the likelihood of another outbreak. 

"Some of the modelling that the likes of Shaun Hendy have done for the Ministry of Health says by the end of the month, our certainty around elimination is still fifty-fifty. That only increases with time and testing," Ardern told The AM Show.

Dr Hendy of the University of Auckland and research centre Te Pūnaha Matatini has provided the Government with estimations of just how widespread the virus is since it arrived on our shores and predictions of what might happen under different alert levels. 

The AM Show host Duncan Garner asked Ardern what would trigger a return to alert level 3 or 4.

"Not just a case," she said. "We've seen a couple of managed isolation facility workers who through screening have been identified as having COVID - that wouldn't be a trigger, for instance... 

"If we have a situation where we have a case that comes up through screening - so someone who goes to their GP or community testing station - and we cannot identify where it's come from, that's a cause for concern. That's where we'd consider it."

The source of a confirmed case on Saturday remains a mystery, though it's believed the patient - who arrived here from India and spent 14 days in isolation, testing negative twice - may have just had an unusually long incubation period, rather than have caught it locally. 

Ardern said the alert level system is designed to respond quickly to potential outbreaks.

"When we're worried about heat, we have fire alert levels. When we're worried about water restrictions, we have levels we move in and out of. "That's what our framework is for. It's a stamp-it-out strategy - when we're worried and we think things are a bit on edge, we may escalate and then we de-escalate."