ANALYSIS:

The emergence of at least two cases in Wellington this morning makes an extension of alert level 4 almost certain.

Level 4 was initially put in place for three days for the whole country, which is set to expire at midnight tonight, with an expected seven day timeline for Auckland and the Coromandel.

The Wellington cases, the lessons from NSW, and the cautious approach favoured by Prime Minister Jacinda Ardern all point towards more time in lockdown.

It is not impossible that the South Island could move to level 3 from tomorrow, but ministers would have to be highly confident that there are no cases there.

This seems unlikely given that many test results from over 1000 close contacts at potential super-spreader venues are yet to come back.

Cabinet will base its decision today on many factors including the number of cases, whether they can all be linked to existing cases, the number and location of close contacts, and the level of testing all over the country.

There won't be any certainty about the size of the outbreak or how large it might become, and ministers will rely on the advice of director-general of health Ashley Bloomfield.

As of yesterday's 1pm update, there were 21 cases - 20 in the cluster, and one more in the community with the Air NZ aircrew member.

Since then, other cases have emerged including a patient who had been in North Shore Hospital.

This morning two positive results - yet to be confirmed by the Health Ministry - are understood to have come back in a family in Miramar, Wellington, and there could be other undetected cases in other parts of the country.

That's because many of the 120-plus locations of interest would have included patrons from all over the country, including some of the tourist attractions that the Devonport tradesman went to in the Coromandel.

High testing numbers around the county would give a good indication of whether there are any cases.

There were almost 13,000 tests processed nationwide on Wednesday, though the Ministry of Health has not provided a regional breakdown.

Wastewater testing will also provide some clues, and so far no results have shown any sign of the virus outside Auckland.

Today Cabinet will consider wastewater results from Thames and the Coromandel that were taken yesterday.

Modelling by the team led by Professor Shaun Hendy estimates there could have been about 100 cases in the community at the time the first one was detected on August 17.

The unknowns about the cases so far include whether there might be more than one chain of transmission.

So far there has been no link between the probable index case - a traveller from Sydney who flew to New Zealand on August 7 - and the 20 cases in the cluster.

Yesterday Bloomfield said 12 of the cases in the cluster are connected, five are thought to be linked to Avondale College, and the remaining three are still being investigated.

There were 362 close contacts identified in yesterday's update, though that did not include those at Avondale College or those who had attended a church service on Sunday - which will push the numbers to more than 1000.

None of them so far were outside of Auckland or the Coromandel, Ardern said yesterday, but some will almost certainly emerge as more cases are detected.

There are still many unknowns about how large the outbreak might become.

More cases are expected, and most of the test results from the venues including SkyCity Casino, Avondale College, and the church service are also yet to come back.

Advice from Bloomfield is based on a public health assessment, but Cabinet will also consider the economic impact, as well as whether there are any cracks in public compliance.

Police Commissioner Andrew Coster has said that compliance has mostly been good, though the threshold for arrests has been lowered because Delta is so much more transmissible.

Finance Minister Grant Robertson has said that the country's economic output is cut by about $1.5 billion a week when alert level 4 is imposed on the whole country.

Ardern has also said movements through alert levels will be staged, and there won't be a lurching from level 4 to level 2 or 1.

If Cabinet decides to move any part of the country to alert level 3, it would be based on a high level of confidence that there are no Delta cases there, and travel restrictions would be required to ensure none could be carried in.

There remains a lot of uncertainty, and these calls are always a finely balanced judgement based on the best information available at the time.

If there are still too many unknowns, Ardern has previously remained cautious and stuck to lockdown until more information - and more confidence about the size of the outbreak- can be gained.

The last thing she wants, as she has said repeatedly, is to yo-yo between alert levels.