COVID-19: Why 24 mystery cases and low testing could see Auckland's alert level 4 lockdown extended

While the number of new cases in New Zealand's Delta outbreak has been plateauing for several days, there are two dozen mystery cases causing concern for experts.

As of Tuesday there are 24 community cases that haven't been epidemiologically linked to the outbreak. There are currently eight subclusters in the Delta outbreak - the two largest are the Mangere church group with 368 cases and the Birkdale social network with 76 cases. There are nine epidemiologically unlinked subclusters. 

Of the 21 new community cases announced on Tuesday, one was a mystery case. The mystery cases pose an issue for Auckland shifting down alert levels because it makes it hard for experts to assess the virus' spread. 

Cabinet will review Auckland's alert level next Monday, but the number of unlinked cases would likely need to be reduced to give officials confidence alert level 3 is the right move.

Earlier in the week Director-General of Health Ashley Bloomfield and Prime Minister Jacinda Ardern said while mystery cases aren't unusual at the start of an outbreak, due to the strict lockdown settings Auckland has been under for several weeks, they are more concerning.  

She said because of the lockdown there are far fewer exposure events than at the start.

"If you're getting them [unlinked cases] later on... then they are more risky to us. If you've had them right at the beginning of your outbreak, that's very different than if you got 20 yesterday. So we assess that, as well."

It's a sentiment echoed by Dr Bloomfield, who warned while fewer daily cases is good, to shift alert levels the number of mystery cases needs to drop. 

"Our most important message today is if you have any symptoms you must get tested, wherever you are in New Zealand," he said on Monday. 

"Our goals are to improve our confidence there is no undetected community transmission in Auckland and to protect the rest of the country from any possible leakage of the virus out of the Auckland region."

And low testing rates aren't helping that. Earlier in the week Aucklanders were warned if they want to see alert levels drop, testing numbers have to increase.  

"So long as we don't know where a case has come from, we cannot tell if we have other chains of transmission in the community that we haven't isolated yet. That's a cause for concern and it is why testing is so important to us," Ardern said. 

On Tuesday there were only 7255 tests processed, with 5852 in Auckland. On Monday that number was even worse with only 4750 tests processed -  the lowest since the outbreak began. 

The rest of New Zealand shifted to alert level 2 on Wednesday.