Assaults on Corrections staff by prisoners have almost tripled in the last decade, even as the national prison population declines.

There were 909 assaults of varying severity committed against Corrections staff in the year to May, a percentage increase of 156 per cent from the 355 assaults in 2013.

It resulted in more than 6200 workdays being lost over that period as staff recovered from their injuries.

The marked increase has occurred while the prison population has regularly declined since 2018 - dropping to 7927 from 10,540 four years ago.

The Corrections Association of New Zealand says the level of assaults is shocking and blames the increase on a lack of consequences prisoners face when they assault a staff member.

Corrections minister Kelvin Davis says staff deserve to feel safe, a point he has "made clear" to those running New Zealand's prisons.

The data, given to the NZ Herald through the Official Information Act, showed annual assaults on staff increased in seven of the last 10 years.

Assaults were defined by three categories; assault - no injury, assault and serious assault.

An assault - no injury included when a staffer was pushed or shoved, an assault was when a physical mark like a bruise was left, and a serious assault was when a bone was broken or a staffer spent two days in hospital.

In 2013, there were 233 assaults without injury, 109 assaults and 13 serious assaults.

In 2022, there were 589 assaults without injury, 287 assaults and 25 serious assaults.

As expected, the number of workdays lost to recovery had increased - jumping from 1015 in 2013 to 6257 in 2022.

The data also included the number of sexual assaults against staff over that time.

Those assaults were more sporadic with eight being recorded in the year to May, 2022, none in 2020 and 2021, 16 in 2019 and 10 in the remaining six years.

More gang members being locked up has been linked to the issue. Photo / NZME

Data specific to New Zealand's 18 prisons was also provided with almost all prisons experiencing increases in assaults since 2013.

Chief among them were Auckland Prison, Mt Eden Corrections Facility and Christchurch Men's prison.

The country's three women's prisons had all observed marked increases.

"They are shocking statistics," Corrections Association of New Zealand union president Floyd du Plessis said.

He cited what he considered a long-standing problem in which prisoners weren't held to account for their actions.

"They don't feel that there's much of a consequence."

Corrections Association of New Zealand union president Floyd du Plessis. Photo / Supplied

There wasn't sufficient data analysing what happened to prisoners after they assaulted staff, but work was being done to extract that information from police, du Plessis said.

However, he said instances of assaults not being followed up were commonplace, referencing an incident that occurred 18 months ago that police had still not decided whether to investigate.

"If we allow that behaviour to continue, at some point when they get released, that thinking and mindset continues.

"It supports and builds on the increase of violence in the population."

While he couldn't quote data, du Plessis said the rising threat of assaults had severely impacted staff recruitment and retention.

Staff at New Zealand's prisons are at a higher risk of assault due to more violent offenders being locked up in recent years. Photo / Dean Purcell

In 2018, a Violence and Aggression plan was initiated to bring the number of assaults down.

Du Plessis believed few solutions had been enacted through the plan.

He wanted more prisoners prosecuted and further training given to staff so they could de-escalate incidents.

In a statement, Corrections Minister Kelvin Davis referenced the "prison crisis" inherited by the Government in 2017 when the prison population had increased beyond 10,000.

Since then, it had reduced 27 per cent.

Corrections Minister Kelvin Davis. Photo / Mark Mitchell

However, he said more serious offenders - including gang members - had been locked up during this time which posed a risk to staff.

"This brings its own challenges, particularly to staff who have to manage a heightened environment where mental health issues and methamphetamine addiction are also increasingly more prevalent," he said.

Davis said staff deserved to feel safe and the Violence and Aggression plan was one way to reduce assaults.

He referenced the $23 million from Budget 2022 given to hire an extra 64 staff across the three prisons facing the most challenges in this area - Auckland Prison, Christchurch Men's and Mt Eden.

That was part of the extra 518 staff who would be hired during the next four years.