New Zealand Prime Minister Jacinda Ardern said the releasing of two women from quarantine before they were tested for Covid-19 was "unacceptable".
The women, who are New Zealand citizens, had flown from London to visit a dying parent and were granted an exemption to leave their mandatory 14-day quarantine early on compassionate grounds.
They then travelled by car from Auckland to Wellington, where they tested positive for coronavirus.
Ms Ardern has assigned a top military leader to oversee the nation's border quarantine measures after the "unacceptable failure".
Health officials said the women had no contact with other people on their road trip.
However, officials said they are contacting 320 people who may have come into contact with them on their flight or in the hotel they stayed at during their time in quarantine.
Before the two new cases were announced yesterday, New Zealand had gone more than three weeks without reporting any new cases and was considered virus-free.
Ms Ardern has advocated tough border measures to prevent another outbreak and has cancelled quarantine exemptions on compassionate grounds while the case is investigated further.
She said she had appointed Air Commodore Digby Webb, the assistant chief of defence, to oversee all quarantine and managed isolation facilities.
