Prison guards are handcuffing prisoners five times more often than they used to and say it's to protect themselves from rising gang violence.
Latest figures show handcuffs were used 962 times in the year to June 2017 - a 400 percent increase since 2009 and over a period where the prison population rose by about 25 percent.
The figures show instances where prisoners were restrained over and above the standard policy for handcuffing, such as when an inmate is transferred between prisons or goes to court.
Corrections chief executive Ray Smith said it was because of a ballooning inmate population that was more violent than ever.
"Most of the violence that occurs in prisons occurs by those people that belong to gangs, not exclusive to them, but they're the most likely perpetrators," he said.
"We do have an environment where we have an intensity in our high-security wings in particular of gang members and the violence that they bring from the outside."
In 2013, Corrections made handcuffs more readily available to staff, after a review found they were not being used enough to prevent violence.
Mr Smith said handcuffing was working because while prison numbers had increased, assault rates had remained stable.
Following a series of random attacks at Auckland Prison's maximum security unit, inmates there are now being routinely handcuffed when taken to the exercise yard or shower block.
Corrections Association president Alan Whitley said the new safety policy came in last Saturday after a staff member was assaulted.
"They were getting the prisoners out of the cells to move them into exercise yards or showers and they weren't handcuffing them.
"At the time you needed a reason to do it but the reason we should be cuffing maximum security prisoners is, they're the worst of the worst."
Mr Whitley said last Saturday was the ninth assault in a two-week period and policy at the prison was changed that night.
But lobby group People Against Prisons Aotearoa said the increasing use of handcuffs should not be celebrated.
Spokesperson Ti Lamusse said handcuffing was a form of violence in itself, and was overused.
"We're still having restraints being used willy-nilly and as a form of deprivation of people's liberty, as a form of violence, that's rising at a much, much faster rate than the prison population and than any potential threat of violence to prison officers."
Victoria University criminology expert Elizabeth Stanley said overcrowded prisons caused stress to staff and prisoners that often manifested in violence.
She said Corrections should look at reviewing its use of restraints and then targeting the most common reasons why staff felt handcuffing was necessary.
She said the New Zealand figures went against international trends and sailed "very close to the wind", with regards to being in line with the United Nations minimum standards for prisons.
"Restraints are often indicators that things are going wrong. In the short term, they can provide that sense of safety but I think in the long term the use of restraints is not the direction we want our corrections system to be going."
But Ray Smith said the main reason New Zealand rates were up, compared to other countries, all came back to gangs.
"[New Zealand has] a particular issue and most prisons I visited internationally don't suffer from the gang-related issues that we do in New Zealand.
"Thirty percent of our prison population is gang affiliated, so that's about 3700 people and sadly those people express violence more regularly."
He said the alternative to not using handcuffs was guards having to physically restrain the inmate, which was much more dangerous.