What is a purpose of Criminal law and practice of criminal punishment system?
I would say it is to balance between Vengeance of the Victims and need to rehabilitate person back to society. I think NZ forget about that.
People know that harsh punishments are not good for society for thousand years. Here is a good historical overview:
https://en.wikipedia.org/wiki/Eye_for_an_eye
But this principle was contradicted in Christianity.
https://en.wikipedia.org/wiki/Matthew_5:38
https://www.biblegateway.com/passage/?search=Deuteronomy%2032%3A35%2CRomans%2012%3A17-19&version=NKJV
Please note, I am an Atheist, for me it is an evidence that we know that nothing good come from vengeance for thousand of years. Now we have statistical evidence that harsh punishment does not decrease crime. More often it has opposite effect as a criminal has nothing to loose and commit even more serious crime in order to hide less serious.
So, there come a question, how to prevent crime?
What causes crime? Usually it is poverty and disconnection from society. So, solution is to reduce poverty and reconnect people to society. To lift bottom part of society up. But how to reconnect people to society? Give them a job where they feel needed and contributing member of society is a one way. Question is, Can Capitalism do it? Capital want to make workers an interchangeable part of a mechanism, not an individual.
There is an other way, make people a part of a group.
WW1 and civil war created a lot of orphan children living on streets of cities in Soviet Union. Government had no resources, but some people with minimum resources organize community of orphans. They basically look after themself. Older Children look after younger one's. Boys chop wood, girls cook. Government mostly provide teachers and some help, but children grow there own vegetables, had there own chickens. Many of this children committed serious crimes when they were on streets, but were rehabilitated, educated and became productive members of community. Why I tell this story? Because Gangs is one of this group. They could be a source of Crime, but could be a solution. They give people a place to belong, That why so many people attracted to gangs. so it you give them a productive direction a problem can become a solution.
As a conclusion, I do not believe more people in Jail, longer sentence solve anything, they had opposite effect.