A high profile defence lawyer says he is shocked by the National Party's plans for the very worst violent and sexual offenders.
National says it would introduce new civil detention orders to keep offenders who are particularly high risk in prison past the end of their sentences.
The orders would apply to as many as 12 offenders over a 10 year period.
The offenders would stay in jail until the Parole Board believes it is safe to release them.
Defence lawyer Greg King says the policy punishes people for what they might do, not what they have already done.
He says it is a kneejerk reaction to the "Beast of Blenheim" case, in which rapist Stewart Wilson is to walk free from prison in September next year.
Authorities are powerless to stop Wilson's release, even though he has said he "doesn't give a stuff" about staying away from children once he is out.
Mr King says the new detention orders came completely out of the blue and should have been opened up to public debate before being set as policy.
However, retired detective Colin MacKay, who led the police inquiry into Stewart Wilson, says a civil dentention order would not be further punishment but protection for those who may be put at risk.
The Parole Board's longest serving member also supports the plan.
Dame Temuranga Batley-Jackson retired last year after nearly two decades on the Parole Board.
She has been present at some hearings for Wilson and says detaining him long term would be a good idea.
"I support that 100%. I am on the side of the victims because some of them never recover from the brutality or whatever that has happened to a family member whereas the offender, he does ten years, 15 years and then he wants to get out."
The Parole Board itself has declined to comment.
