A Kiwi disease modeller says pausing the travel bubble between New Zealand and New South Wales was not an overreaction, and authorities "in fact should have moved a little bit faster".
It comes after an Australian infectious disease specialist said he would not have shut the bubble in the first place.
The New Zealand Government paused quarantine-free travel with the state from 11:59pm on Thursday, after a Sydney man in his 50s and his wife tested positive for COVID-19 in the community on Wednesday afternoon - more than 24 hours earlier.
Genome testing showed it was a similar strain to a person who was in quarantine after returning to Australia via the US, but authorities believe there could be other cases in the chain of infection. The bubble is shut until at least 11:59pm on Saturday.
University of Auckland's Shaun Hendy said on Saturday it's likely more cases will soon pop up in the New South Wales community
"Although we know the genomic link to someone in their MIQ system, we don't know how the virus passed from the MIQ case through to this case and that tells us there are probably other cases out there in the community in New South Wales."
He said the fact more cases may arise is why "it's been important that we pause the bubble".
"The large number of people who have come in, in the last few days, would put a strain on our system in terms of having to contact those people and give them good information."
Between 5000 and 6000 people have returned from New South Wales since the cases were identified.
Dr Ashley Bloomfield told The AM Show on Friday it's unlikely any returning New Zealanders will have been in contact with the positive cases, but caution is being exercised regardless, with all returnees being chased up.
Dr Hendy told Newshub it's a "challenge" for New Zealand authorities to contact such a large number of people which is why the bubble should have been paused faster.
"It makes the difficulty of communicating places of interest that they might have to pay attention to those individuals.
"Hopefully we won't see any other cases but it certainly is a challenge for us dealing with so many people at once."
He adds the likelihood of the bubble opening on Sunday is dependent on New South Wales' testing capacity and test results.
"I think it'll depend on how much testing they can do if they really ramp up their testing over the next few days, and we know they can do this in New South Wales, they can run a very large number of tests there.
"If those tests are coming back negative then it may just be something that the link is not identified but, because of the large scale of the testing effort we can be sure that there's not a large cluster that's present."
Hendy said it was "prudent" of New Zealand to close the bubble until more is understood about Sydney's situation.
However, Professor Peter Collignon from Australian National University said considering the current risk level, he would not have closed travel with New South Wales over the new community cases.
"At a level of one in a million, I would think that's too low level of a risk to draw it all up, so to speak, but if there were a lot more cases then there is an argument to stopping people travelling freely until we get a lot more people vaccinated," he told RNZ.
"I would actually think if we're not finding any more cases in New South Wales, then particularly if there's no more community cases where we don't know the source, yes I think it would be worth firing it up again because I think that actually means the level of risk is very very low."
Dr Hendy said anyone travelling into new Zealand from Australia over the past week should stay "vigilant".
"I just emphasise that if people have come back into New Zealand over the last week or so, and they've been through New South Wales, that they should be paying attention to the locations of interest that the New South Wales authorities are releasing and just making sure they haven't come into contact with these cases.
"And also anyone who's travelled into New Zealand from Australia should be just monitoring and being super vigilant should they develop any symptoms and if they do getting a test."