Violence against prison guards appears to be getting worse, despite a drop in the number of inmates.
There were 355 reported assaults resulting in injury in the last 12 months to May, a 49 percent increase on the same period the year before.
But the prison population has dropped by more than 150 people.
Corrections officers are the front line of defence when dealing with some of the country's worst offenders.
In the past year, prisoners assaulted guards 895 times.
Five-hundred and forty of those assaults involved punches, kicks and shoves but caused no injuries.
Three-hundred and thirty-one assaults caused non-serious injuries from violence such as biting or gouging, where x-rays or stitches might be required.
And a further 24 were classified as serious causing injuries which required overnight hospitalisation and extended periods of ongoing medical care.
Corrections Association president Alan Whitley said the trend had continued upward since 2013 and there was not enough being done by the department.
"We were quite shocked when we saw it.
"We've been watching it rise for the last sort of 18 months to two years, but this year's increase is just totally unacceptable," he said.
"There is a lot more violence in the crime, and it's the top end of crime that is coming to jail now.
"There are a lot of people on community-based sentences, so what you are seeing in prison are the top end of the offenders and there's quite a lot of violence in most of the crimes that are bringing people to jail now."
Whitley said the union was also worried not all assaults were being reported to police as per the process.
The rise in attacks was despite the fact the prison population had dropped.
There were around 150 fewer people incarcerated than last year and only about 1000 more than back in 2013 when there were far fewer assaults.
The number of assaults from May 2019 until May 2020 was 152 percent more than the total in 2013.
Whitley attributed the rise to the growing proportion of remand prisoners, who were waiting to be sentenced.
The majority of these prisoners are held at Mt Eden Prison, which has one of the highest number of assaults compared to other prisons in New Zealand.
"You've got the most instantly volatile people in there, they are coming in off the street, they can still be high on drugs or alcohol.
"They're a very volatile mix, the prisoners in there."
JustSpeak director Tania Sawicki Mead described the growing remand numbers as a crisis.
"In many cases, there's an argument that there's a miscarriage of justice for so many people to be held for so long before their cases have been heard.
"I think immediately that creates extreme tension for people who have been held there because they have such little, if any access to all of the programmes that are supposedly offered by the prison system," Sawicki Mead said.
Being trapped in that process could be a deeply stressful and unhelpful environment, she said.
Corrections chief custodial officer Neil Beales agreed the rising remand population was one of the problems, but bigger numbers of gang members in prison was another factor.
He said younger members from the Crips and Killer Beez were committing some of the most violent acts in prison.
"Completely unprovoked without warning, a prisoner will walk out of their cell, or they'll be being escorted by a member of staff, there doesn't appear to be any tension or hostility and then there'll be a randomised attack on that member of staff.
"We see that often it's cowardly.
"It's a king hit from behind or it's just a quick turn and punch.
"These things are happening in our prisons right now."
Former prisoner and president of the Canterbury district Howard League for Penal Reform, Cosmo Jeffery, said overpopulation and double bunking in cells designed for one person increased the aggression of inmates.
The population had only just levelled out after years of increasing, he said.
"It has been growing, but the space has not gotten any greater, generally speaking.
"So you are cramming more and more people into smaller spaces, and you'll just get aggression.
"It is an inhumane system, and I just know that the more people you're crowding into the spaces, the more you're going to get violence."
Neil Beales said in 2013 the Corrections department worked on staff safety, but he admitted it needed to do more in this area.
"We have to somehow learn how to deal with that better.
"I'm not too sure what that looks like right now, it is clear that we need to revisit that work, we need to go back to the drawing board."
He said it didn't mean the work in 2013 had failed.
"It just means that the environment has again changed around us. The context has changed around us and we need to be able to respond to that."
Beales said 75 percent of prisoners were incarcerated for violent crimes.