The number of prisoners held on remand over the period from June 2017 to June 2027 is projected to nearly double from 3,000 to 5,400, the Ministry of Justice says.

The ministry has released its annual Prison Population Projections, for 2017 to 2027. It says the projected increase in the custodial remand population is driven by a continuation of the increase in inflow as well as a continual increase in the length of time people spend on remand.

Total prison population - 10,300 in June 2017 - is projected to continue to increase over the decade by 4,100 places, to 14,400 in June 2027.

The projection shows an ongoing increase in the sentenced population by 1,700 places, from 7,300 in June 2017 to 9,000 in June 2027.

"This is driven by an expected increase in the number of more serious cases passing through the system as well as people in prison being released on parole at a later point," the ministry says.

The projections are prepared from the latest available data and reflect assumptions on the future levels and trends in prosecutions, convictions, and sentences. The ministry says the assumptions for modelling were agreed through workshops with policy and experts from the Ministry of Justice, the Police, the judiciary, Corrections, Treasury and the Crown Law Office.

The report says the Justice Sector Leadership Board was consulted on the 2017 projection scenarios at their 7 November 2017 meeting, and approved a projection that would reach 14,400 in 2027 as the most likely projection based on current system settings.

Assumptions used in projections

Looking at the assumptions used, the projection says the proportion of people starting custodial remand has been increasing every year for a decade from 5.5% in 2008 to 10.4% in 2017. "It was agreed that this proportion will likely continue to increase over the next 10 years, although the rate of increase will gradually decline over time."

The proportion of people convicted has remained stable for some years at around 76%. The ministry says this proportion is likely to remain constant over the next 10 years.

The proportion of convicted people who receive a prison sentence is increasing. This has increased every year for a decade from 7.6% in 2008 to 12.0% in 2017. The ministry says this will likely continue to increase over the next 10 years, "although the rate of increase will gradually decline over time".

The length of prison sentences has remained stable for some years at around 650 days, and the ministry says this length is likely to remain constant over the next 10 years.

The proportion of sentence served is increasing - from 75.1% in 2014 to 77.0% in 2017. The ministry notes that before the Parole Act 2002 the proportion was 53.3%. "It was agreed that this rate of increase is likely to continue over the next 5 years and then remain constant for the following 5 years.

The ministry says there is more uncertainty about how two other assumptions will track in the future.

The number of new prosecutions entering the justice system is gradually increasing, by 12.1% since 2014. This followed a decrease of 28.4% from 2009 to 2014. The ministry says this increase has been concentrated in more serious crimes which are more likely to receive custodial remand and prison sentences. There is some uncertainty around how long this increase will continue.

The average time on custodial remand has been rapidly increasing - since 2014 it has increased from 60.1 days to 71.5 days. "There is uncertainty around whether this rate of growth will continue or slow and stabilise."