More than 450 murderers and sex offenders are in jail, and five have so far served more than 25 years.
Corrections Department figures show 315 prisoners are serving life sentences, mostly for murder; a further 137 are sentenced to preventive detention. Most of the preventive detainees are sex offenders.
People sentencedto life imprisonment or preventive detention before August 1987 are eligible to be freed on parole after seven years, or 10 years if sentenced after that date.
Department spokesman Graeme Vercoe said that between 1996 and 2000, the Parole Board freed 67 life or preventive detention prisoners. They served an average sentence of 10.94 years.
The board says 39 who were sentenced to seven years or more in prison and were paroled between 1996 and 2000 were recalled to jail for parole violations.
Only five inmates had served more than 25 years. The longest-serving inmate was jailed in 1967 for sex offences. More than 200 had been serving sentences of seven to 15 years, with 15 jailed for 15 to 20 years and one for 20 to 25.
National Party corrections spokesman Brian Neeson said the figures confirmed the public's complaint that criminals were not being jailed for long enough.
They also showed the average sentence in recent times for the most violent crimes was 12 to 14 years.
"It confirms judges are taking no notice of what the public wants. The public concern must be listened to when a sentence is being pronounced."
Sensible Sentencing Trust spokesman Garth McVicar said many of the serious offenders being paroled had more than 200 convictions.
"They've made their choice to be a career criminal, so we should make it easier for them to stay inside."
New legislation will make offenders sentenced to more than two years in jail eligible for parole after serving a third of their term.
Those sentenced to a shorter term will be eligible after serving a year.
Murderers who planned their crime, are extremely brutal or who kill more than one person can expect a life sentence, with a minimum non-parole time of 17 years.
