If prisoner numbers continue to climb and there are not enough beds to go around inmates could find themselves sleeping on mattresses on the ground, Corrections Minister Kelvin Davis says.

And Corrections Association national president Alan Whitley said that could soon be a reality because there was likely to be more prisoners than beds before proposed upgrades are completed.

The news follows the Government announcement to scrap National's mega-prison plan for Waikato's Waikeria prison and instead build a new unit featuring a purpose-built mental health facility.

In an interview with Newstalk ZB's Heather du Plessis-Allan, Davis revealed the Corrections Department had contingencies in place if numbers continued to grow.

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"They are not pretty plans.

"Corrections is looking at options and talking about what we need to do. They could be mattresses on floors and things like that."

But he wasn't expecting that to happen, he said.

The new plans were "just the start" and there would be more initiatives that would be looked into.

"We have given ourselves 15 years to reduce the prison population by 30 per cent and it will take all of those 15 years in order to do that."

He said at the moment people who were unwell were being punished and that was not fair.

Whitley said the last thing Corrections Officers wanted to see was prisoners sleeping on the ground but he believed it was a reality they would have to deal with.

He believed prisoners would be living in emergency accommodation before the Government's rapid build for 976 inmates was completed in by mid-2019.

"The rapid builds will give us respite but before that will be the issue.

"We need new accommodation and we need it quickly, it must be sound and secure but we need it quickly."

Once prisoners went into emergency accommodation it would put "huge pressure" on Corrections officers and could result in more trouble with more people.

"You are forever moving prisoners, and it's the physical area the prisoners are allowed in is where the problems happen."

The change would also spell more overtime for the already hard-worked officers, he said.

Davis said the Government was aware of the bulging prison network, so had approved the construction of 976 more beds at five different prisons around the country to help relieve the pressure.

"Decades of rushed policy and reactive decisions have gotten us to this point today. First off we need to find a way to deal with the numbers of people currently coming through the prison gates."

Corrections chief executive Ray Smith today said about 50 per cent of the new beds would be double bunking.

He also confirmed psychiatric staff would be permanently on site in the new mental health facility at Waikeria Prison.

Greens MP and party spokeswoman for Corrections Golriz said the ministers comments were only meant if there had been no change in policy and if inmate numbers continued to climb.

"I understood the minister's remarks to be more if the prison population continued to grow at its current rate, which is why we are adapting a new prison population policy.

"Our position is that we don't want you to be in contact with a prison in order to get mental-health help."

Under the National led Government they were "failing to keep anyone safe", she said.

"We keep telling victims we are getting tough on crime but we have 50 per cent reoffending rate at the current level of imprisonment."

She did not see inmates sleeping on mattresses as a possibility.

"If we are bold, brave and honest as policy makers and shift policy then numbers will go down."

National said the move was a cop-out by the Government and put the public in danger as there would not be enough beds to house serious criminals.

National's Corrections spokesman David Bennett said by not having that capacity it put New Zealand residents at risk because there were going to be a lot of serious criminals that were going to have to be let out of prison.

"So whether it's a small or a big prison isn't the defining factor ... it's whether you've got capacity in the system. And they won't have the capacity now."

He said although a specific mental health unit was a good idea, it would only skim the surface on the huge number of prisoners who were affected by mental-health issues.

"A vast majority of prisoners have mental-health issues and so just picking out 100 of them that are the more severe cases doesn't actually provide the rehabilitation we need for a vast majority of prisoners."

WAIKERIA'S NEW PRISON

• A new 500-bed unit, plus a new purpose-built 100-bed facility for mental-health patients

• The new unit will replace the old high-security upper jail

• 62 per cent of prisoners diagnosed with a mental health or substance disorder in last 12 months

• Earthworks on new construction to begin in August

• Facility to be completed by early 2022

• Rolleston, Tongariro, Rimutaka, Christchurch Womens' and Christchurch Mens' to share 976 extra beds between them