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Christ's College began contacting all its old boys in December last year after a former student told the Royal Commission into Abuse in Care about abuse and bullying by fellow students in the 1970s.
In the apology, school board chair Hugh Lindo said many of those contacted since then were also survivors of abuse and they received "many emails from old boys describing their own traumatic experiences at College".
They described a culture of bullying in all its forms, "a culture where they were victimised, a culture where they were not safe, a culture where corporal punishment was condoned and a culture where they could not ask for help and, if they did, it was not there", he said.
There was likely a much larger group who might never come forward, Lindo said.
"To this group, we acknowledge your suffering, acknowledge that for you Christ's College was a frightening place to be and that we could not keep you safe. For this we are sorry."
The college was now confronting that aspect of its history and for many, that had taken too long, he said.
"We are now putting the interests of our old boys above those of college. This was not always the case."
As part of its complaints process, the board allowed for redress and said that may involve compensation.
Lindo spoke to RNZ's Susie Ferguson: