There are fears people testing positive after visiting Middlemore Hospital could be "the tip of a bigger iceberg" of Covid-19 cases circulating in the community.
The Ministry of Health announced on Saturday night three people had tested positive after turning up at Middlemore Hospital for unrelated reasons, including a five-month-old baby.
It's not known how any of the three were exposed to the virus.
Te Pūnaha Matatini Covid-19 modeller Professor Michael Plank said it was concerning there were cases popping up without a clear link to the current Auckland cluster.
The mathematics professor said the likelihood of people turning up at Middlemore for unrelated reasons and happening to test positive for Covid was, "on the face of it...very, very small".
"The fact that it's happened on several occasions does suggest that there could be a hidden cluster out there, potentially."
That was based off the information currently available to the public, but he wasn't ruling out officials finding a link between cases through contact tracing efforts or genome sequencing.
The latest locations of interest update - at noon today - showed 133 events involving 96 locations. There is one small change from 1oam — a slight adjustment for a September 2 visit to Pak'nSave Clendon Park, where the time window reduced to 3:40pm-4:10 pm.
Cabinet is meeting tomorrow to decide on alert level settings for Auckland.
While it had been hoped the city would be able to move to level 3 soon, Plank said based on current information that would be "very risky".
"If there are still a significant number of mystery cases that don't have an epidemiological link, that will be a cause for concern because if we are looking at the tip of a bigger iceberg here then moving to level 3 will obviously allow that to grow much faster."
Even a few undetected cases could see the outbreak spiralling in level 3.
"The danger would be, you'd move to alert level 3, that would allow the virus to start spreading but it could be a couple weeks before you notice," he said.
That was because of the the lag between getting infected and getting tested.
"By that time we could have quite a large number of cases on our hands again so it could really set us back."
Auckland's mayor Phil Goff told RNZ the rise in cases - following last week's dip - was a concern.
He did not expect the city would be shifting down a level at Monday's announcement, despite excellent vaccination rates - including 26,000 people in Auckland getting their jab yesterday.
"It looks likely that there may be a further delay in coming out of alert level 4. But really there's no alternative if we want to stamp out the transmission.
"We've made the sacrifice now for nearly four weeks and we need to stay the course."
Targeted testing key to moving out of level 4
The Ministry of Health has said it wants to see 7000 tests a day. Plank agreed with that number but said the right people needed to be targeted.
Current numbers include MIQ workers and essential workers like truck drivers who need to cross Auckland's borders. That was important but he wanted everyone with symptoms to get tested - even if they were mild, and even if the person had been vaccinated.
"We know the vaccine reduces the likelihood that you're going to get infected and show symptoms, but it's not a guarantee," he said.
Despite the mystery cases Plank believed our level 4 settings were right - borne out by the solid downward trend in cases over the last week.
"But [level 4] does need to be accompanied with really good testing to make sure that we do find everything that's out there. Then our contact tracing system can kick into gear and do its stuff - because that's an important part of the response as well."
The most optimistic scenario was finding an epidemiological link between the current mystery cases.
"If that happens, and we also get the number of cases infectious in the community down...and we have good testing so we've got confidence there's not something missing out there, then we potentially could be in a position to drop the alert level."