One of the world's leading prison reformers says there is no way social problems can be fixed by locking people up in jails for longer.
Baroness Vivien Stern sits on several international think tanks and United Nations organisations analysing prison policies, and is in New Zealand for a lecture tour.
She told Radio New Zealand's Sunday Morning programme that many countries have been increasing the number of people they incarcerate, and this has not helped combat crime.
The baroness says there are many going to prison in New Zealand who do not need to.
"New Zealand were locking up 197 per 100,000 and Germany 83 per 100,000 and if you walk around the streets of Berlin or Wellington you don't feel threatened in either place at all - you have to ask what's going on?"
She says high prison populations are increasingly unaffordable and can cause social problems.