Christchurch has recorded two community cases of COVID-19, breaking the city's 358-day run without the virus.
The New Zealand government has opted against sending South Island's biggest city into lockdown, with Chris Hipkins saying there were "no major exposure events".
"Close contacts have been identified and are self-isolating ... we'll continue to closely monitor the situation," he said.
Health officials reported 89 cases on Thursday, including 84 in Auckland, four in the Waikato and the two Christchurch cases.
The Christchurch cases were found on Wednesday night, a development the city's Mayor Lianne Dalziel labelled "the wake-up call we've all been dreading".
The cases are both from the same household, and the link to NZ's Delta outbreak is believed to be recent travel from one of the cases from Auckland.
Both cases were unvaccinated, and did not make use of QR codes to sign in on their whereabouts, but were co-operating with contact tracers who continue interviews on Thursday morning.
"They'd been back in Christchurch for several days, potentially up to a week," Mr Hipkins told Radio NZ.
"This is someone who did have a negative test prior to travel ... my understanding is that they'd been unwell for a while.
"Given there are a number of days when the person was infectious, It's quite possible there will be more (close contacts to test)."
Ms Dalziel said it was "extremely disappointing" the pair weren't signing in, as is mandatory in NZ.
"This has been the wake up call we've all been dreading and a reminder of the importance of being absolutely vigilant," she told Radio NZ.
According to the Canterbury district health board, 89 per cent of eligible locals are partially vaccinated and 69 per cent are double-dosed.
Another case was picked up on South Island earlier this week, further north in Blenheim, but that is yet to result in further spread.
Until this week, Christchurch enjoyed a run of almost a year without COVID-19.
The last confirmed community case in the Garden City was in November, and the last confirmed community transmission was in May last year.
There are 37 New Zealanders in hospital with the virus, including five in intensive care.
Australian Associated Press
