Epidemiologist Professor Michael Baker says case numbers of the Delta variant of Covid-19 are tracking as expected, but it's too early to tell how high they will go.

Professor Baker said it remained possible that those infected with the strain had travelled to other parts of the country and it could spread outside of Auckland and Wellington.

He said things would become clearer once more test results started to come in.

"We've heard from the modeller a range of estimates from 50 to 120 cases. I think that's still a pretty realistic range. This is obviously a middle range estimate because it can be easily a lot more if one of those cases when they were infected went to a super spreading kind of event."

He said it was too early to be optimistic the cases were confined to Auckland and Wellington.

"I don't think we can guarantee that. The other known contacts have been distributed all around the country so those people I imagine will be having the tests over the weekend and early next week so the results from those may not be available until Monday, Tuesday, Wednesday."

He said wastewater testing would also be very valuable information.

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The immediate shift to alert level 4 restrictions was an excellent move by the government and would have limited the spread of the cluster.

"The problem always is the outliers and we know the lockdown is very effective but it does have some weak spots. One of those is the essential workers which are 15 percent of the population. If, for example, one of them was infected, they could potentially present the virus more widely.

"That's why it's so important that they wear masks and are vaccinated.

"The other potential weak point in this is that we know the demographic who were infected were a young age group and they have big social networks and they may sometimes feel less engaged with the response."

He said the mask mandate would help everyone get out of lockdown faster.

"This is not forever. This is just for this period when there is a risk of a circulating virus."