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".
"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.