The discovery of Covid-19 cases in Waikato changes what the government will need to consider when it decides today whether Auckland can move out of alert level 4, epidemiologist Michael Baker says.
Three household members of a Mt Eden remand prisoner with Covid-19 have tested positive for the virus.
Two are students at Mangatangi School, and one had symptoms while there.
The household is just south of the level 4 border in the Kaiaua/Whakatīwai area.
The person on remand was bailed to the Waikato on 8 September.
The government is due to announce any possible alert level changes this afternoon and it is unclear how the development in Waikato will affect its decision.
Baker told First Up having community transmission in an alert level 2 zone where there were fewer controls is very concerning.
"I think it does change things. We know this decision has been in the balance because while the total case numbers have been on a bit of a plateau for the last two weeks we really needed to focus on these unexpected cases.
"The problem in the Waikato is that it's alert level 2 and at that level there's very few controls on transmission of the virus.
"These young people were at school, and at that environment at level 2 all they have is a bit of physical distancing and we know that's not enough to stop transmission of this virus. So they could easily have infected other people at this school."
"Contact tracing ... can stop the virus, of course, but at alert level 4. At alert level 2 unfortunately it is so infectious and has a shortened incubation period that means quite hard to get ahead of it with just contact tracing."
Students may disperse and have further contacts, he said, so an option would be to bring that part of the Waikato into the Auckland alert level.
"I'm sure there will need to be some consideration about whether to somehow extend the level 3 or level 4 boundary to include this part of the Waikato."
Deputy Prime Minister Grant Robertson told Morning Report that among advice Cabinet is looking for is on whether both Auckland and part of Waikato could be in alert level 3.
The Waikato cases did not necessarily tip the balance against moving Auckland out of level 4 lockdown, he said.
"We don't believe we have widespread community transmission - what we have is a series of clusters," he said. That meant when it got into a household there were a reasonable number of cases.
'It's very, very likely the infection will have spread further'
Robertson said preliminary indications showed there may be an epidemiological link to one of the current clusters.
Covid-19 modeller Professor Michael Plank said while that was partial good news, the virus is still likely to spread in the Waikato.
"Just because cases are linked it doesn't mean they haven't spread it in the community.
"The news that household members have tested positive and have been infectious in the community, it looks like for around a week, does mean it's very, very likely the infection will have spread further in those Waikato communities.
However it was good news that the case hadn't "popped up from nowhere" and been spreading for a longer period.
"I do think the government will have to look at alert levels for this area - it may be they can move the alert level boundary and have parts of north Waikato in the same alert level region as Auckland.
"Given the fact the virus has likely spread in that community in level 2 conditions it could potentially have spread quite quickly and there could be quite a large number of cases there already."
Officials needed to see a lot of test results to gauge how far the virus may have spread, he said.
Plank said it would be be risky to move Auckland down alert levels at this point.
"Although the outbreak is mostly contained we are still seeing spread between households that's proving quite hard to stamp out."