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The Ministry of Health says there are two positive Covid-19 cases in Christchurch from the same household.
Covid-19 Response Minister Chris Hipkins says the case has been in Christchurch for up to a week.
One member of the household had recently returned to Christchurch from Auckland.
The case tested negative for Covid-19 before travelling to Christchurch, and got a second test once they became symptomatic, Hipkins said.
Hipkins said he cannot rule out a snap lockdown in Christchurch, but they are still waiting on more information before making that call.
He was expecting to receive a public health risk assessment on the cases later this morning, which will allow a quick decision about a potential lockdown, possibly at today's 1pm briefing.
He told TVNZ the two people who have tested positive for Covid-19 in Christchurch are unvaccinated.
The ministry says the local public health unit is identifying close contacts and exposure events, including any locations of interest.
A public health risk assessment is being done this morning and there will be an update after that.
Hipkins said the pair had come into contact with at least one other household in Christchurch.
"The [cases] have not been particularly good users of the Covid tracer app," Hipkins told TVNZ of the pair.
He took the opportunity to once again call on the public to get vaccinated.
He said it was now not if the virus would get to those unvaccinated, but when.
Hipkins told Morning Report "The nature of their contacts in Christchurch will be established during the morning and then we'll have a bit more information to share, but you know, these cases came in quite late last night.
"My understanding is that they've been unwell for a while so they could have been symptomatic and infectious for a period of time whilst back in Christchurch and so that's one of the things that the contact tracers will be working on as a as a top priority this morning - to identify exactly what the nature of the potential exposure in Christchurch is."
The impact of their low use of the Covid Tracer app "ultimately would depend on exactly how much they've been out in the community, you know if they've been unwell, they may well have been staying at home for much of that period of time.
The pair seemed to be co-operating with authorities so far, he said.
Christchurch residents are rushing to get tested this morning.
The Orchard Rd clinic near Christchurch Airport was full of cars at 10.30am.
Mayor hopes lockdown not necessary
Christchurch mayor Lianne Dalziel hopes cases in the city aren't widespread.
"This has been the wakeup call that we've all been dreading," she told Morning Report.
She says it's extremely disappointing to hear the people who tested positive haven't been scanning in regularly.
It's also disappointing to hear they are unvaccinated, she says.
She hopes a snap lockdown won't be necessary.
Local National Party MP and former party deputy leader Gerry Brownlee told Morning Report with any luck the cases in Christchurch will be contained quickly.
"It's a bit alarming that they were tested before they left Auckland and were clear."
He hopes there is a "sensible analysis of what the risks are" before a decision is made regarding the alert level.
He said vaccination wasn't always easy to get in the city but he hopes the numbers of people getting vaccinated continues to grow.
Epidemiologist Michael Baker told Morning Report there's some inevitability about the coronavirus spreading through the country, but the South Island cases are still a shock.
He says New Zealand has a good contact tracing system so if it's just one case and a household contact it should be "manageable", and there may not be a need for alert level change. However the public health risk assessment will be helping determine that, he says.
"I think the wider question is how many other cases are incubating in the South Island."