Health authorities have ordered a major overhaul of how assaults on mental health workers are recorded after a Herald on Sunday investigation.
The Auckland District Health Board shake-up followed one nurse quitting as it was too dangerous to work at acute mental health unit Te Whetu Tawera.
Nurse Lauren Meraw left the country after writing to Health Minister Tony Ryall saying staff and patients were "at serious risk of harm".
The inquiry prompted the health board to check statistics, conceding 131 assaults were committed in 12 months against staff.
The revelations sparked criticism from opposition health spokesman Grant Robertson and Public Service Association general secretary Richard Wagstaff.
"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.