Christchurch is in a "risky period" after two Covid-19 cases emerged in the city, University of Canterbury mathematical modeller Professor Michael Plank says.
The Ministry of Health said in a statement on Thursday morning they were notified on Wednesday evening of two positive cases from a single household in Bishopdale.
One member of the household, a woman in her 50s, had recently returned to Christchurch from Auckland. The ministry said the person travelled to Auckland to provide childcare and was there for five days.
The woman tested negative for Covid-19 before travelling, and returned a second negative test before they returned to Christchurch on October 15.
However, while the woman wasn't considered infectious when she travelled back to Christchurch, she became symptomatic on October 18 then later tested positive for the coronavirus.
The second case is the woman's partner - a man in his 40s. He is a truck driver and was working for four days while in their infectious period.
He started experiencing Covid-19 symptoms on October 20.
Both cases are unvaccinated against Covid-19 and did not use the Covid Tracer app.
"There's obviously now a number of locations of interest and they're spread quite widely across Christchurch so it's going to be really crucial to see the results of the testing today and tomorrow for everyone who's been to those locations of interest and, of course, anyone who has symptoms," Plank told Breakfast on Friday.
"The concern is that because these cases weren't regularly using the app there could be other locations that they visited that haven't been reported, perhaps they've forgotten about some of them, so we are in a risky period at the moment I think."
On Thursday, Director-General of Health Dr Ashley Bloomfield said he had not advised the Government to put Christchurch into lockdown despite the two cases because there weren't yet indications of widespread transmission.
But Plank said he wouldn't be surprised if the alert level changed.
"No alert level change so far but the results of that testing are going to be crucial and if it turns out the virus has spread more widely in Christchurch we could still see an increase in the alert level," he said.
"We're in a risky, a fairly uncertain period at the moment, I mean, the fact that they became unwell and then subsequently stayed at home is obviously a good thing because that means they haven't been out-and-about in the community a lot during their infectious period.
"But on the other hand, of course, we know people can spread the virus before they actually get sick and so because of the time periods involved that could potentially be a couple of weeks ago and they only had to pass it on to one or two other people and then the virus has had, kind of, two weeks to spread."
Plank said more could be done to tighten border restrictions following this exposure, though, "because this is going to be an issue for us over the next couple of months while we get those vaccination rates up and we work towards that 90 per cent target and beyond".
He suggested rapid antigen testing, which takes between 15 and 20 minutes to record a result, for plane and ferry travel.
"It won't catch every last case but they would provide an additional safeguard that will just slow down the speed at which the virus can make its way around the country."
Plank also suggested vaccination requirements for travel as well, particularly flights where people are in close quarters with one-another.
"We're going to be having vaccination requirements to go to the gym, to go to the hairdressers, so I think we could look at extending that requirement to flights as well," he said.
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