Marist College has revealed 11 cases of COVID-19 have been confirmed at the school, with more positive tests likely in the coming days.
In a video on the school's Facebook page on Thursday, Stephen Dallow, chair of the school's board, said seven teachers and four students had tested positive for the virus.
"We suspect we will have more tomorrow," Dallow said in the video, which was addressed to the school's community.
"We have an outbreak in our school. We didn't choose this, it is our reality. We understand how frightening this will be for some."
Included in those confirmed to have COVID-19 was the school's principal Raechelle Taulu, Dallow said.
"Anyone who is worried their daughter has had direct contact with any of the positive cases will be contacted by the Ministry of Health. This is part of the track and trace process and it's really important that we get this process right."
Because of the nature of the outbreak at the school, all students and staff were being treated as close contacts, he said.
"We are sorry that this has happened to your daughters and to our Marist community
"Please be assured that we could not have predicted that this would happen. We don't know where the first COVID-19 case came from and we may never know."
The outbreak at the Auckland school was one of a number of clusters of the virus in the country.
On Thursday, director-general of health Ashley Bloomfield announced there were 78 new confirmed or probable cases of COVID-19 here, bringing the total number to 283.
The figures came as Kiwis faced their first day of lockdown after the nation's alert level was raised to 4 - the highest level - at 11:59pm on Wednesday.
The new level means all non-essential businesses have to close and everyone in the country must self-isolate for at least four weeks.
Dallow said no students or staff at the school, with the exception of the principal, would be named for privacy reasons.
"This is a challenging time. The senior management and the board are with you. We're just as scared as you"