He said staff are constantly working to ensure the prison provides the safest environment possible for staff and prisoners.
"Our staff manage some of New Zealand's most dangerous people in a complex and challenging environment.
"Over 75 per cent of the prison population have convictions for violence in their offending histories, and more than 90 per cent have had a lifetime diagnosis of a mental health or substance abuse disorder," he said.
A spokesperson said the victim was not the man charged over last month's massacre in Christchurch.