Figures released under the Official Information Act reveal the average number of prisoners held in New Zealand's prisons went from 5765 in 2001-02 to 8567 in 2011-12.
University of Canterbury sociology professor Greg Newbold said while the prison population had remained relatively stable over the past two years, prison forecasts remained difficult to predict. Although authorities could make "reasonable predictions" based on crime rates and how many people were sent to prison, it was difficult to predict how many prisoners would be released by the New Zealand Parole Board.
The board based its decision mainly on two factors: the seriousness of the offence, and the prisoner's progress in prison "and that is something you can't factor in".
Dr Newbold said parole had got tougher in recent years, particularly since Graeme Burton murdered Karl Kuchenbecker in 2006 while on parole.
"[Parole] gets criticised every time it makes a mistake, but it releases thousands of prisoners each year and is bound to make a mistake from time to time."
He said the board had become "risk averse", and regularly recalled people for minor bail breaches and put them back in prison.
"The Parole Board is bloody tough, but it is understandable given the public outcry when a parolee reoffends," he said.
Tougher sentences were also being handed out, and this was coupled with the number of remand prisoners climbing, as courts were under pressure from justice groups.
"We are going down the same track as America, and the danger is America is an extremely vindicative society."
A Parole Board spokeswoman said, "The board is required, as its paramount consideration in every case, to consider the safety of the community and the board will not release an offender on parole unless it is satisfied that, in its view, on release the offender poses no undue risk."
The risk of an offender was assessed over time, she said.
"It is important to remember that an offender cannot be detained in prison longer than is consistent with the safety of the community."
Corrections assistant general manager Maria McDonald said forecasting the prison population was carried out by the Ministry of Justice, with the department using those figures as a guide.
The decade after 2001-02, the number of female prisoners more than doubled from 252 to 521, although this was down on the 555 females locked up the previous financial year.
The rise in the female muster was projected, and the facility at Auckland Region Women's Corrections Facility was opened in 2006, she said.
During the same period, the male prison population had increased from 5513 (2001-02) to 8587 (2011-12), and again this was fewer than the 8156 male prisoners the previous year.
Ms McDonald said the significant increase in remand prisoners (907 in 2001-02 to 1894 2011-12) was because "Corrections manages remand prisoners that have been deemed unsuitable to be at at large or bailed in the community".
"This is decided by a judge."
The department had recently adopted an objective of reducing reoffending by 25% by 2017, resulting in 600 fewer reimprisonments, 4000 fewer reconvictions, and 18500 fewer victims each year.
