Experts are warning there could be dozens of people out there infected with COVID-19 in addition to the five that have already been found.

New Zealand was plunged into a level 4 lockdown overnight after the discovery of a single case on the North Shore on Tuesday. Four more have since been found - all linked to the first case, and all the highly infectious Delta variant.

University of Auckland disease modelling expert Shaun Hendy told The AM Show on Wednesday morning the average person with the Delta variant spreads it to about six other people.

"It could be a lot less than that or it could be significantly more. That's why it's important people check those locations of interest."

The first case, a North Shore man in his late 50s, visited Coromandel while potentially still infectious - the Ministry of Health quickly able to list numerous locations he visited in the past week, thanks to his use of the COVID Tracer app. 

Just how big the cluster might be depends on how close the first case is to the border, assuming that's where the virus came from. No direct link has been found yet - suggesting there might be other links in the chain.

"The big question at the moment is whether we're looking at the tip of the iceberg," said Dr Hendy. 

"It's not so much who this person may have gone on to infect, although that's obviously important; it's who infected him? Does that mean there are a large number of other cases out there at the moment? That's the crucial variable…

"If there's a plausible link to the border, that's telling us that there might be fewer cases out there at the moment. If we can't spot that link to the border, then we're looking at a situation similar to what we faced in August in Auckland last year - at the time we discovered that cluster, around 50 people had been infected at that point."

Delta is at least twice as contagious as previous strains of the SARS-CoV-2 virus, which causes COVID-19. But not everyone spreads it as easy as others, as Wellington recently found out when a Delta-positive visitor didn't appear to cause an outbreak. 

READ MORE

• Four new cases - Delta Strain in the community
• Health officials investigating one new case in the community in Auckland

"Because there’s no clear link to the border, it’s also possible the virus has been spreading undetected for a significant period of time," said Michael Plank, modelling expert at the University of Canterbury and Te Pūnaha Matatini. 

"In that case there could easily be more than 100 people infected by now and a strict lockdown is our only available option."

But there are reasons to be hopeful. 

"We do know the wastewater testing in the North Shore was negative last week, so that's a good piece of evidence that maybe it's not a very large cluster out there that's been going for a couple of weeks," said Dr Hendy. "We would have expected the wastewater testing to pick that up last week, were we looking at a very large cluster. So that's good news."

And the quick move to level 4 will also go a long way to preventing a New South Wales-style outbreak. 

"One thing we've learned from watching Sydney is that half-measures can quickly lead to disaster," said Dr Plank. "It’s better to go hard at the start and then relax than the other way round. With Delta there are no second chances."

The vaccine was developed to fight the original strain of the virus found in Wuhan at the end of 2019. While it offers strong protection against illness from the Delta variant, vaccinated people are still able to carry it and infect others. Studies suggest the infectious period is shorter however, thanks to the vaccine's quicker suppression of the variant's viral load, compared to an unvaccinated person's immune system. 

The first case found on the North Shore was not vaccinated. His wife was, and she's so far tested negative. 

"Presently we just do not have enough of the population already double vaccinated to take any risks," said Lesley Grey, University of Otago senior lecturer and public health expert.

"Can everyone who was not scanning into places visited in the last five days recall exactly every place visited? The message is clear - we cannot be complacent. Scan, hand hygiene, mask up, and if you feel unwell with any covid symptoms at all, and isolate until you get your test results."