Experts are warning the current outbreak of COVID-19 in New Zealand could be "potentially grave", with transmission perhaps going back a couple of weeks and known cases visiting high-risk locations.
Nearly 1000 people were at Auckland's Bar 101 on Friday night while an infected person was there, and 230 were potentially exposed at a church service on Sunday. Other indoor locations with large gatherings where people may have been exposed include Event Cinemas on Queen St, Avondale College, Glenfield Mall, Pak'nSave Wairau and Sky City Casino.
The Ministry of Health has dozens of locations of interest on its list, going back to August 3. Just nine cases have been found so far - thankfully all linked, but there's no known connection to the border.
"It's good news that we're finding these cases and so far they're all linked," University of Canterbury disease modelling expert Michael Plank told Newshub. "But that' because we're testing close contacts of cases - it's still going to be the next few days when we really start get a sense of how big this outbreak could be.
"It's certainly possible we could be looking for a large number of cases out there that we haven't found yet."
Director-General of Health Ashley Bloomfied said modelling suggested between 50 and 120 cases.
"My strong expectation is that we will get more cases," he told The AM Show on Thursday.
The Delta variant appears to be better at infecting young people, overseas studies have found. New computer modelling by Canadian researchers has also found young men might be driving undetected transmission.
In September last year, before Delta arrived, University of Otago epidemiologist Michael Baker warned young people might be driving the spread of COVID-19 thanks to their "few symptoms and big social networks".
"It's certainly possible we could be looking at an outbreak of that scale, particularly with the news that there are a number of cases of people in young age groups, locations of interest in busy, crowded locations that are high-risk for transmission," said Dr Plank. "We could see case numbers go up over the next few days before we're able to start to bring this outbreak under control."
Early in the COVID-19 pandemic there were concerns after an infected person attended a concert by American metal band Tool. Despite that, and the fact lead singer Maynard James Keenan also had the virus at the time, no one else got infected. But the Delta strain of the virus is at least twice as infectious, and can spread more easily via the air.
"It's certainly looking potentially grave," University of Otago infectious disease specialist David Murdoch told The AM Show.
"The further back it goes, the more worrying it is. What we're hoping is... there's a good connection with the border. We're keeping our fingers crossed."
If a connection to the border can't be found, it will be difficult to figure out where the outbreak started, or how long it's been bubbling away without being detected. If it remains elusive, Dr Plank said it's likely the South Island will have to spend longer than three days at level 4.
"We don't know yet where this man caught the virus from or what the link to the border is. We really have no idea, and the virus could be actually anywhere in the country.
"So it's really important that people everywhere in New Zealand actually follow the rules because it's the actions of everyone over the coming days and weeks that will determine how this pans out in the long-term. What we really want to avoid is a situation like Sydney is in, which could absolutely happen here in New Zealand."
The good news, Dr Murdoch says, is that it's likely a New South Wales-sized outbreak would have been found by now. But asked how worried he is on a scale of one to 10, he said "high".
"We've all been concerned about the complacency that has crept in... it just shows you how important it is to do the scanning, to do the tests."
Dr Plank said the level 4 restrictions - now with added mask mandates - will "dampen down the spread", giving contact tracers a chance to get ahead of the spread.
"Then it's all about just getting all of those cases in isolation as soon as possible. That's what we need to do to get this outbreak contained."