The path to a $1 billion mega-prison was paved with "assumptions" and a "best guess" to predict the numbers of prisoners, new Ministry of Justice documents reveal.
But repeatedly inaccurate prison projections blew out estimates of how many prisoners could be expected and left politicians scrambling to find somewhere to put them.
Documents revealed through the Official Information Act show officials and politicians struggling to get to grips with a ballooning prison population.
From 2014, the actual number of prisoners consistently defied carefully worked out prison population projections needed to plan for the size of prisons, staff and budgets.
It culminated in the development of the Waikeria mega-prison plans which were scaled up from a proposed 1000-bed increase to a 3000-bed, $1 billion facility.
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The new Government, which has pledged to cut the prison population, has said the mega-prison will not go ahead but has yet to explain what it will do with the growing number of inmates.
The change is illustrated by predictions in 2012 picking the prison population to be around 8000 people in a decade. The current prison population is around 10,600 with predictions now that it will rise to 12,200 by 2026.
A December 2016 briefing to officials from NZ Police, Ministry of Justice and Corrections NZ spelled out how frequent "tough on crime" law changes and the way they were brought in created increasing uncertainty around projections.
They were told: "The work involves deciding upon assumptions regarding the impact of changes and the continuation of trends. The assumptions are a best guess — they will often be wrong."
Officials told former Minister of Justice Amy Adams the growth in the prison population was "unexpected" and New Zealand's incarceration rate was "high by international standards".
She was also told that judges and the NZ Parole Board seemed to have become more "risk-averse", putting prisoners away for longer and keeping them behind bars. They were unsure if it was down to "get tough" talk from politicians or a reaction to one-off, high-profile events such as Graeme Burton committing murder on parole.
Victoria University criminologist Dr Liam Martin said quick-fire "tough on crime" law-making since 1999 had introduced so many variables that officials were in an almost impossible position.
A recent review of New Zealand's criminal justice system by the Prime Minister's Chief Science Adviser Sir Peter Gluckman said the "tough on crime" policies had made New Zealand less safe by damaging people who needed help and creating hardened criminals.
Martin said the models of projections included complicated mathematical formulas but "there's a lot of moving parts" and "it's just guess work".
Ministry of Justice sector insights manager Anton Youngman said projections were made on "current legislation, policy and practices" at the time.
The assumptions — what would happen because of current known factors — were agreed on by the Ministry of Justice, the Police, the Judiciary, Corrections, Treasury and Crown Law, he said.
Youngman said the process was studied and endorsed with some recommendations for change by an outside reviewer in 2015 and 2017.
Corrections NZ deputy chief executive Jeremy Lightfoot said the justice sector was working together to "safely manage the impact of a higher than projected increase in the prison population in recent years".
"The prison population has grown by almost 30 per cent in the past five years, for reasons that are outside of Corrections' control, including changes in legislation, policy and practice."
Lightfoot said remand changes, prisoners serving more of their sentence in prison and an increase in longer sentences for serious crime had driven numbers up.
He said it was Corrections' job to manage those sent to prison and "we do not have the option of turning people away".
There were current plans to add 900 extra beds to the prison network by the end of 2019.