Kitset cells are being put together for 360 inmates as our prison population hits its highest ever levels.

Our growing numbers of inmates will challenge the incoming government with election promises to reduce those behind bars even as Corrections confirm levels have "grown beyond what was forecast".

The number of people in prison on Friday was 10,481. It's almost 2000 more than three years ago and more than 1000 inmates above projections.

To deal with burgening numbers, Corrections has pushed through construction of a new $1.5 billion prison in the Waikato, set up a team to help inmates get bail in the community and increased the number of inmates forced to share cells.

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It has also turned to the kitset cells with parts built off-site then shipped to the prison to be put together.

All efforts are factored into dealing with a prison population that continues to increase with extra space being made available for an additional 2205 people by 2021.

In contrast, Labour has come to power with a policy to reduce inmate number by 30 per cent over 15 years.

Deputy leader Kelvin Davis, who pre-election was the party's Corrections spokesman, could not comment on the issue yesterday with the party yet to assign ministerial portfolios.

National prisons commissioner Rachel Leota said increasing prison numbers were "heavily influenced by external factors outside Corrections direct control".

That included law changes, decisions in the courts, policing trends and crime levels, she said.

Changes to Bail Act in 2012, in response to the slaying of Christie Marceau, contributed to a huge increase in the numbers of prisoners held on remand. Marceau, 18, was killed by a teenager who was bailed on charges of kidnapping and assaulting her.

Other law changes have seen fewer inmates paroled and inmates kept in prison with longer sentences.

National Commissioner Rachel Leota said the steel and concrete-lined "modular accommodation units" would be double-bunked.

There are two 120-bed units set for Rolleston prison in Canterbury and another at Tongariro in the central North Island.

"A modular unit is a new fit-for-purpose block that is constructed offsite, and fitted together onsite. The units will accommodate low- to medium-security prisoners and be ready to use next year."

The kitset prison cells are quicker and cheaper to produce than a new prison, with most of the building work taking place outside the prison

The rise in inmates numbers since July 2014.
The rise in inmates numbers since July 2014.

Leota said they had been used by prisons in Australia, the United States and United Kingdom to provide "a durable, well-built, cost-effective, safe, secure and humane option".

She said there was traditional building work taking place at Arohata, Hawke's Bay and Christchurch Men's prisons to further expand capacity as part of a "carefully considered approach" to deal with the prison population.

Leota said other work included helping prisoners get past barriers to electronic bail and home detention by help fill in paperwork or finding an acceptable address.

Justice advocate Roger Brooking said the first step to dealing with prison numbers was to repeal the Bail Amendment Act.

He said it was a "knee-jerk reaction" from "tough-on-crime" pundits to which the National Party had responded.

"Now there are so many people being put into (prison on) remand who don't need to be there."

Corrections figures show the number of prisoners on remand has gone from around 1800 inmates three years ago to just over 3000 people.

Brooking said the definition of "short" sentence also needed to change, pushing it from two years to five years. Inmates serving "short" sentences are automatically released after serving half the sentence.

He said any public concerns about safety could be met by using savings on keeping people locked up to improve funding of drug and alcohol services in the community and to provide better services for released prisoners.

The pressure on Corrections has seen the total number people behind bars lift beyond 10,500 recently - an 18 per cent increase in inmate numbers in three years.

The rise has mainly occured in the last three years and accelerated in the last 12 months.

If it kept increasing at the same rate, then the prison population would top 12000 by December 2020.