Some of the worst criminals sentenced to preventive detention are being freed after serving on average only 11 years in prison.
Corrections Department figures show preventive detention might not be as tough as the public perceives it to be.
A total of 22 inmates sentenced to the stiffest penalty the courts can provide have been freed on parole in the past seven years, 17 in the past three years.
The average time served was 11.7 years, despite the sentence enabling offenders to be kept locked up indefinitely.
Legislation introduced in 2002 lowered the minimum age for the imposition of preventive detention from 21 to 18.
However, it also reduced the minimum non-parole period from 10 to five years.
Since 2001, 166 offenders have been sentenced to preventive detention in New Zealand.
Corrections assistant general manager Maria McDonald said an offender sentenced to preventive detention had to be considered for parole once they had completed their minimum non-parole period.
The Parole Board's paramount consideration when deciding a prisoner's release was community safety.
"Note that anyone released on parole from preventive detention is recallable for life, she said.
"This means they are on parole for life and if they present any undue risk then we can apply to the board to have them recalled."
The figures have dismayed Brian Brown, whose daughter, Natasha Hayden, 24, was strangled by Michael Curran in January 2005.
Mr Brown was appalled some of the most dangerous criminals were being released into the community after serving fewer than 12 years behind bars.
Curran was found guilty of Ms Hayden's manslaughter in mid-2006 but, while awaiting trial, he murdered two-year-old Aaliyah Morrissey.
He was sentenced to nine years jail for Ms Hayden's manslaughter and life imprisonment for Aaliyah's murder, with a non-parole period of 20 years and six months.
"Curran didn't get preventive detention. "It seems judges don't give out those sentences lightly," Mr Brown said.
"It's for the worst of the worst, and yet we seem happy to have 22 of them back in the community.
"Someone could argue releasing 22 offenders was OK, but would you want them living in your street?"
Mr Brown said prison reformers highlighted the cost of keeping offenders locked up but the alternative was often untenable.
"It may cost us $100,000 to keep dangerous criminals behind bars but if they're released they can create mayhem for a family and what cost do you put on that?"
Hamilton barrister Roger Laybourn said releasing people from preventive detention could be a "positive sign".
Prisoners weren't being locked up and forgotten, he said.
He said preventive detention inmates still served, on average, more than the minimum 10-year non-parole period given for murder.
"Lengthy sentences are utterly pointless if they don't go hand in hand with some robust rehabilitation," he said.
"In the vast majority of cases people have to be released eventually and if they haven't been rehabilitated then society is letting down the potential victims whenever they are released."
BY THE NUMBERS
Number of prisoners sentenced to preventive detention and released on parole from 2006-2012: 22
Number of prisoners sentenced to preventive detention from 2006-2012: 96
- © Fairfax NZ News