Department of Corrections figures show that only one of 21 people released from preventive detention has been recalled to prison in the past six years.
A leading lawyer says the statistics show the system is working but the mother of one victim says the justice system needs to be very careful about releasing some offenders.
Preventive detention - usually imposed in cases of sexual or violent crime - means a prisoner can potentially be jailed indefinitely and can be recalled to prison at any time for the rest of their life.
The figures show 21 such prisoners were released between 2006 and 2012 and they served an average of 12 years before being granted parole.
The recalled prisoner was held in prison for seven days for breaching his parole conditions before being re-released.
The Howard League for Penal Reform says the figures show the Parole Board is being cautious and only releasing offenders who don't pose a risk to the public.
The league's patron, Nigel Hampton, says the figures show the system is working as it should. He says the low recall rate vindicates board decisions to release prisoners sentenced to preventive detention.
But the Sensible Sentencing Trust says the non-parole period on many preventive detention sentences is too short and it is too difficult to recall a prisoner once they are granted parole.
"We see a lot of these offenders causing a lot of mayhem out in the community, as well as causing more trauma to their original victim," spokesperson Garth McVicar says. "Recall seems to be very, very difficult.
"The police, I believe, are doing a great job but we believe legislation is failing us and putting the community at risk."
Rachel Kitson, the mother of Vanessa Pickering, who was murdered by Malcolm Chaston in Christchurch, says the justice system needs to take extreme care when granting violent offenders parole.
Prison guards tried to raise the alarm about Chaston in e-mails to the Sensible Sentencing Trust in 2007, saying he had talked about wanting to hurt women badly.
