"You have to hope they would know more about it. The management of ADHB have let the staff down," Robertson said.
Wagstaff said: "Any manager of mental health services knows it is potentially very dangerous work. The first thing to solve any problem is to find out the scale of it."
College of Mental Health Nurses representative Dr Daryle Deering said complex events were behind assaults on staff.
"There isn't any nurse in New Zealand who would go to work expecting assault."
The ADHB's clinical services manager Fionnagh Dougan said senior mental health managers outside the unit did not need to know how many assaults took place - only the most serious ones.