Assaults on prison guards by inmates have increased by more than 100 in the last year.

But the Department of Corrections say that a three-year staff safety plan will "directly contribute to making prisons a safer environment".

The Department of Corrections Annual Report was released at 4pm today and revealed that in the 2015/16 year just under 20 staff were seriously assaulted by inmates while on the job, and more than 450 minor and non-injury assaults were also reported.

In 2014/15 there were less than 10 assaults on staff classed as serious and just under 400 minor or non-injury incidents were recorded.

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The report comes two days after three Corrections officers were assaulted - one of them seriously and requiring hospital treatment - by a group of prisoners at Auckland Prison.

The report, signed off by Corrections chief executive Ray Smith, revealed that the prison population has more than doubled since 1996.

Back then just 4079 people were incarcerated in New Zealand, compared with 9798 as of September this year.

As a result of that increase assaults on staff had also risen.

Better reporting of incidents by staff had also contributed to the increase.

The rate of prisoner-on-prisoner assaults remained about the same, with 45 incidents regarded as "serious" occurring in 2015/16.

Most of the assaults in prisons are gang connected.

"Prisoners with gang affiliations were said to commit a disproportionate number of assaults in prisons," Corrections confirmed.

"In 2015, gang affiliated prisoners were responsible for just over half - 55 per cent - of assaults despite comprising 29 per cent of the prison population."

Corrections bosses said staff safety was their priority.

"We are working continuously to improve safety and security at all prison sites. Every instance of violence is taken seriously, and we will not tolerate prisoners using violence against others.

"A prisoner responsible for violence will be held to account for their actions.

"While no assault is acceptable, it is an unfortunate reality that they will occur from time to time as we manage some of New Zealand's most difficult and challenging citizens."

The report stated that "Identifying and understanding the drivers for assaults is critical to reducing levels of violence in prisons".

"The Staff Safety Plan increases our potential to predict violence in prisons and improve our capacity to respond through additional staff training and interventions," the report states.

"This plan directly contributes to making prisons a safer environment, through the creation of tools to support the early identification of risks or heightened tensions in prisons.

"Corrections' first priority is to prevent violence... Staff use tools such as the Prison Tension Assessment Tool as an indicator of the level of tension within a prison unit."

Waikeria Prison High Security Unit exercise yard. Photo / Michael Craig
Waikeria Prison High Security Unit exercise yard. Photo / Michael Craig
Auckland South Corrections Facility in Manukau. Photo / Dean Purcell
Auckland South Corrections Facility in Manukau. Photo / Dean Purcell

The prison population at a glance

Every few months Corrections releases information about its prison population.

As of September 2016 there were 9798 people behind bars in New Zealand prisons.

Of those 6.8 per cent were female.

The majority of prisoners were in jail for violent offending with 37.6 per cent of the total population made up of people sentenced on violence offences, followed by 20.1 per cent for dishonesty and a further 20.1 per cent for sexual crimes.

The ethnicity of inmates was mostly Maori, with 4989 inmates followed by 3153 inmates who identified as European, 1108 Pacific Islanders and 466 Asians.

The most common age of prisoners was 20-29, with 3357 inmates falling into that bracket.

There were a further 365 inmates under the age of 16 and 16 over the age of 80.