New laws aimed at keeping violent criminals off the streets would have done nothing to stop at least 20 of New Zealand worst murderers.

The Corrections Department has confirmed the law would not have applied to double murderer Graeme Burton who killed Paul Anderson in Wellington in 1992 and Karl Kuchenbecker in 2007 in Wainuiomata. It would also not have applied to William Bell, who killed three people at the Mt Wellington RSA in 2001. He was sentenced to a minimum of 30 years. Despite the men having extensive criminal histories, none of their previous offences would have put them three strikes category.

Kim Workman from the group Rethinking Crime and Punishment says the situation raises questions about picking out future offenders.

"The idea that by increasing the sentence you are going to redress crime is a bit of a myth. The difficulty is that even the experts can't work out which ones are likely to reoffend."

Mr Workman says violent offenders are spontaneous and unremorseful, so harsher penalties do not deter them. He believes the three strikes law will not do much to weed out violent offenders.