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Assistant Commissioner Bruce O'Brien said across New Zealand five schools on Tuesday and a further 13 yesterday were targetted by what was believed to be computer-generated phone calls threatening harm to those in the school.
Rolleston College was among the schools targeted yesterday. The Rolleston campus was briefly locked down on Thursday afternoon and police attended. Phone threats were also aimed at schools in Kaikoura, Geraldine, Marlborough and Ashburton.
"We're looking at all of those incidents at the moment and we've been in contact with schools to provide reassurance," O'Brien told the AM show.
He said the calls were all similar in nature mentioning there were explosive devices planted in the school.
"The threat is to cause harm to students and staff at school. They are of a very similar nature and mention explosive devices so they are designed to concern disruption that we've seen play out in the last couple of days."
O'Brien said the phone threats were going through the main school line.
"There's a similarity between the threats that have been made across the 18 schools. We're taking it very seriously. We've got our cybercrime teams working with local investigators and we will hopefully identify the person or persons as soon as we can.
"We believe that the calls are computer-generated, that they are masked by a computer, but we are looking into the similarities of the different events.
"These threats are of a nature that have caused enough concern by the school to go into lockdown or evacuate the school."
He said police were focused on finding the origins of the calls, examining whether they were being made here or offshore.
"We're considering all our options at the moment. If you look back at 2016 we had a similar cluster of threats made to schools around New Zealand and that had originated offshore so we're looking at options at the moment and working with our international partners."
O'Brien said the latest slew of robo hoaxes were designed to cause concern, harm and disruption.
"We obviously saw harm and concern caused to students and parents this week who were sitting at home hearing about these evacuations."
He praised schools for the way they had responded to the threats and the level of reassurance they had given their school communities during this distressing time.
O'Brien said it was concerning that somebody would think this was an appropriate way to conduct themselves and police were looking to hold whoever was responsible to account.
Auckland's Sacred Heart College, one of the schools to receive threats, saw 1200 students lose a day of school, with 130 staff sent home and 100 boarding students locked in the dining room for four hours.
Principal Patrick Walsh said the school also had to postpone parent-teacher interviews, a number of students had their respective sports games cancelled and several students had also been given counselling because of anxiety issues related to the incident.
"And keeping in mind that this is in the context where there's increased gun violence in New Zealand, there's a stabbing every day in Auckland and then people are mindful of what's happening in the United States.
"So it's a very anxious time for parents and students."