Prime Minister Jacinda Ardern says the number of women coming into the corrections and justice system has been rising for some time.

Corrections says it did not foresee the recent, rapid rise in the female prison population.

The rising population forced the Department of Corrections to re-open a self-contained unit at Rimutaka Prison, north of Wellington, last year to house an additional 112 women.

And the head of corrections says there's no clear sign the rise in serious offending by women is going to stop.

STACY SQUIRES/STUFF Corrections facilities have been at their limit but the department has contingencies in place.

In September last year, 800 women were in jail – both remand and sentenced prisoners. This compared with 672 in September 2016, a 19 per cent increase in just 12 months.

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In September 2012, only 511 women were behind bars, meaning the female prison population has gone up more than 56 per cent in just five years.

The total prison population had also risen at its most rapid rate over the past few years, but the number of men behind bars has not gone up quite as much as the female population. In September 2012, there were 8112 men in prison. By September last year, the number was 9670 – an increase of 19 per cent.

The female prison population rose nearly three times as fast as the male population.

On Thursday, Department of Corrections chief executive Ray Smith said the rising female population was a "huge problem".

"No one predicted that would happen."

Female prisoners committed similar violent crimes to men, such as burglaries and robberies. There were also women who had breached their bail or parole conditions.

"The drivers for the female population are not very different to the drivers for the male population, but for one factor: and that one factor is sexual offending," Smith said.

KEVIN STENT/STUFF Minister of Corrections Kelvin Davis has not yet made a decision on whether to go ahead with the 2000-bed Waikeria Prison. Meanwhile, the contractual deadline is looming.

There was also a growing proportion of female prisoners who were affiliated with gangs, either as patched members or associates.

Meanwhile, drugs, especially methamphetamine and psychoactive substances continued to exacerbate the problem.

Prime Minister Jacinda Ardern said the rise in the female population was just one part of the wider story in corrections at the moment.

"It's the reason we've been told that unless something changes, we'll be building new prisons every few years. No-one in New Zealand wants to see that happen, when we have a pretty static crime rate.

"It doesn't make sense. It's up to us to make sure we balance safety and security for our communities while having a sensible justice system."

DRUGS:

The issue of drug addiction and dependency had continued to grow, Smith said.

"It has a huge impact on people's lives. It affects the psyche in a way that makes people reckless, violent, dangerous, unpredictable, and unable to manage their own lives. "

It was now taking two to three weeks for people to come down off those drugs once they're in prison. In the past, it would take about one week.

"People come in and they're in very poor state.

"Then there is a long downstream effect on people's mental wellbeing, they have extreme anxiety, depression, and other psychosis on the way out of coming off these substances."

The department was in the process of training its 20 detector dogs to sniff out psychoactive substances, in order to ensure they were not being smuggled into prison.

While drug infiltration was low (about 4 per cent), the ingredients in psychoactive substances were constantly changing, making them difficult for dogs to detect.﻿

GANGS:

Smith said 30 per cent of the prison population was affiliated with a gang, either as a patched member or an associate.

That percentage had risen rapidly in the past decade. Previously, about 15 per cent were affiliated with a gang.

While corrections did not gather data on exit, Smith said some people also joined gangs during their time in prison.

"I meet a lot of people who want to escape the gang lifestyle, they want to do better for their children and are worried about the repercussions of leaving gangs, and it's encumbent on all of us to try and help those people find a way out."

There have been recent reports of new Australian gangs setting up chapters in New Zealand, following the deportation of violent criminals to New Zealand from Australia.

"I'd be concerned about the advent of any criminal gangs. I say that because I see the damage.

"I see young men who do things because they're part of gangs. And they don't really know why they've done them. And these things they do might cause them long prison sentences, long absences from their families, and if you've got kids, your kids need to look up to someone who's achieving something with their life... I worry for the children of these people."

GROWING POPULATION:

There have been ongoing concerns about the rapidly rising prison population, which had led to a lack of beds and an increase in double bunking (about 40 per cent).

The concern over the rise in the population comes as the government faces pressure to make a decision on whether to go ahead with its $1 billion, 200-bed prison at Waikeria.

Meanwhile, the government has also put in place a 15-year target to reduce the prison population by 30 per cent, in pace of the former government's shorter-term Better Public Service target, which required corrections, justice and police to work together to achieve the goal of 10,000 fewer violent crimes by 2021.

Police has adopted this target as a departmental one, meanwhile justice and corrections have not, but continue to work with police on their target.

The population is almost at 11,000, the highest it's ever been. In 2016 the prison population exceeded 10,000 for the first time, and has continued to grow since then.

However, Smith said the forward projections showed growth was expected to slow.

Throughout the population rise, corrections had continued to put in place contingencies, work on new builds, and retro-fit facilities to accommodate double bunking, which was now at 40 per cent.

"I think we've done a phenomenal job. There hasn't been a day when we've missed a beat...

"We've been certainly stretched, and at times, we've been right at our limits, particularly with the female population."

Smith emphasised New Zealand prisons had never been, and legally could never be, overcrowded.

He was confident corrections would not get to "zero beds" scenario.

However, if there was something like a natural diaster, fire or riot, which took out facilities, there were things the department could do to house those people elsewhere inside secure facilities.

That would include putting stretcher beds in gyms and corridors, and while it wasn't ideal, there was further capacity in the case of an unforeseen event.

"One of the things people don't understand about rehabilitation in the corrections context is we're trying to rehabilitate minds. It's not like a broken leg, or a broken arm, or trying to get someone up walking again... You're trying to rewire people's thinking."

"What you don't want to do is get into a whole range of short-term measures that undermine long-term success. We didn't get here overnight, it's taken a long time to get to the point where we have this very large prison population.

"We've got to recognise what's driving that, and understand it, and start to treat those symptoms," Smith said.

During the elect committee hearing he did say he expected the government to set some shorter-term targets the relevant departments would have to meet along the way to achieving a 30 per cent reduction of the prison muster in the next 15 years.

Smith said he expected the coalition government's plans for a further 1800 cops by 2020 to have a downstream effect on the prison population, however, he would not elaborate further.

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