A school spokeswoman said students had been evacuated from the campus due to a "bomb threat".
"As police are yet to determine the veracity of the threat, the school is evacuating everyone from campus as a precaution."
The school will be closed on Friday, May 14, the spokeswoman said.
Parents eventually received an email from the school just after 10.30pm alerting them to the "threat".
A concerned parent said an email wasn't acceptable.
"To simply get a notification via email is not acceptable, not for something as serious as an alleged bomb threat. For the fees we pay, the school should have the ability to send out an automated text message."
Students and staff who board on-site are being relocated and parents were told they can collect their children throughout the night. A police and security guard presence has been increased.
A parent told the Herald: "All the boarding students are being evacuated."
St Peter's School in Cambridge is the largest co-educational boarding school in New Zealand.
The school has been in the news this week after the abrupt departure of executive principal Dale Burden and his wife, and deputy principal Yevette Williams.
It has now emerged that anonymous bullying complaints were made to workplace bullying watchdog WorkSafe in term 1, the school board has confirmed.
In an email - which also acknowledges the slow release of information to parents and caregivers - released yesterday, board chairman John Erkkila revealed further details about the absence of Burden and Williams.
He said WorkSafe had contacted the board in term one regarding anonymous complainants concerning allegations of workplace bullying and that WorkSafe was making inquiries.
"Following a meeting with WorkSafe, we worked to understand the best course of action and the board engaged two independent investigators."
"Those investigators have sought to understand what may or may not have occurred. Those investigations are not yet complete, but I want you to understand that this issue is being taken incredibly seriously.
"And I want to understand, if any wrongdoing is found, how that was allowed to happen and how we go about ensuring it doesn't happen again," he said.
Erkkila also acknowledged that "feedback from our school community that we haven't told you enough and that we haven't told you soon enough".
"As a board, we have legal obligations and for that reason it has been a difficult environment to navigate. But I also recognise the need to be open as possible with our school community is important."
He added that the investigations were still under way and the board "cannot and will not pre-determine that".
"But if it is deemed that workplace bullying has occurred, I want you to know that it will be addressed.
"I cannot and will not allow bullying of any kind in our workplace. Being on the receiving end of bullying behaviour is incredibly damaging to a person's mental health and it will not be tolerated, now or ever.
"If there has been or if there is still workplace bullying happening, we need to know so it can be stopped."
The update comes after WorkSafe confirmed yesterday it was making inquiries about "staff wellbeing" at the school amid the absence of Burden and Williams.
The pair left the school sometime towards the end of term one.
"WorkSafe has been notified of concerns about staff wellbeing at St Peter's School and is making inquiries," a spokesperson said.
