The government does have a responsibility to detect and punish criminal wrongdoings, but it also has a responsibility for education and health for example, and it has limited money. It is proper that the government balances all of its fiscal responsibilities. Fiscal crises have been the impetus for reducing the prison population in many states of the USA.
In addition, it has been said that the best criminal policy is social policy, and The Opportunities Party wants to save money by putting fewer people in prison and spending more on good social policy, which will mean New Zealand will need fewer prisons.
Secondly, prison raises its own moral issues. In New Zealand, prison falls hardest on disadvantaged New Zealanders. If you are poor, need help with reading and writing and suffer from mental illness or drug addiction, you are much more likely to end up in prison.
Then there is the shameful fact that while only 15% of the general population, Maori are 51% of the prison population. Like Indigenous in Australia and Canada and African Americans in the United States, Maori are massively overrepresented in prison.
Finally, most prisoners are parents and their imprisonment has a negative affect on their families and communities. In New Zealand, the children of prisoners are about five times more likely to go to prison than children with parents who will have never been in prison. When large numbers of parent-aged adults cycle through stays in prison, extracted from their families and communities, then their families and communities are negatively affected in many ways, including damage to social networks and relationships and labour markets.
 Superu (June 2015) Improving outcomes for children with a parent in prison at www.superu.govt.nz/sites/default/files/What%20Works%20Children%20of%20Prisoners.pdf.