A political bidding war between the main parties to prove who is tougher on crime has led to New Zealand having one of the world's highest rates of imprisonment, an international study has found.

The comparative study of 11 countries' justice systems found New Zealand's is racist and punitive and imprisons people at a rate second only to the United States.

The United Kingdom Audit Office study looked at New Zealand, Australia, the US, France, Canada and the Netherlands among others.

It found New Zealand imprisoned offenders at a rate of 199 for every 100,000 of the population, second only to the US at 748. That is 25 per cent higher than England and Wales, and 33 per cent higher than Australia.

Between 2005 and 2008 New Zealand's rate of imprisonment rose by 15 per cent despite the crime rate only rising by 4 per cent over the same period.

Surprised at New Zealand's high prison population, the authors conducted their only case study on New Zealand.

They found despite New Zealand being ranked first in a global peace index which looked at issues such as corruption, violence and crime rates, only 57 per cent of New Zealanders felt safe, a rate comparable with Iran and Bulgaria.

New Zealand also remanded more offenders in custody at a rate of 43 per 100,000, compared to 30 in Australia and 25 in the UK.

The study found that since the mid-1980s a "bidding war" between National and Labour over who was tougher on crime had led to a dramatic increase in the prison population.

"This means the use of harsher sentences without reference to their impact on crime trends or generally agreed opinion about their penal effectiveness," the report said.

It also found Maori suffered from "structural inequality and systematic criminal justice system bias".

Maori represented 15 per cent of the general population but 51 per cent of the prison population. That was higher than the rate of African Americans, who made up 13 per cent of the US general population and 42 per cent of its prison population.

Kim Workman of Rethinking Crime and Punishment was interviewed for the study and said the justice system's treatment of Maori was a matter of "national shame".

Green Party justice spokesman, David Clendon, said another study by Otago University had found Maori were more likely to be questioned by police, arrested, charged, sentenced and sent to jail.

The government needed to be courageous enough to put resources into community-based programmes, he said.

Labour's justice spokesman, Charles Chauvel, said nine out of 10 prisoners had drug, alcohol, literacy or mental health problems.

"Yet for sentences under a year there are no programmes available so that person can get treated.

"So all that happens is they go to jail and learn to be a better and more agile offender."

Workman said the study showed there was no correlation between imprisonment levels and the crime rate. "What it is telling us is that we won't get any significant crime reduction unless at the same time we have a strategy in place to reduce imprisonment levels."

The government was building a new 960-bed private prison at Wiri in South Auckland despite there being 1500 spare beds in the system, he said.

However, Corrections Minister Anne Tolley said Wiri was necessary because the prison population was 360 ahead of forecasts and there needed to be spare capacity as well as more beds in Auckland.

The new prison would be better equipped to run rehabilitation programmes than older prisons.

The government was developing policies to reduce reoffending rates and was particularly focused on youth crime, she said.