New Justice Minister Andrew Little is proposing ambitious policies to drop New Zealand's prison population by thirty per cent over the next fifteen years.

Little is warning that if the Government does nothing, projections show that the number of inmates will increase by fifty per cent over the coming decade,

He plans to tackle the issue by looking at the way bail laws are being applied, greater material and mental health support for inmates, and plans to reduce Maori offending while Labour is in office.

Little said he, as well as the Corrections and Police Ministers, are in talks about ensuring inmates are assessed when they enter prison, and get the help they need so they're less likely to re-offend.

Half of all prisoners have mental health problems or issues with addiction, and they don't get enough help, he said. Others are eligible for parole but don't have access to the addiction or counselling courses in prison that they need to undertake.

"I think we all agree that if you commit an offence or a crime, you have to pay your penance, you have to account for yourself," he said. "But if you're unwell, or if you've got problems that if they were fixed with stop you from reoffending, then it makes sense to go down that track."

"We have a lot of people in prison at the moment who, if they were able to get access to courses and things that are going to help them, would be safe to the community and could be released from prison."

He also proposed a sentencing council to look at the sentences being handed down, and work out ground rules that underpin what the law says - so there's some consistency across the court system.

"We don't know whether [sentences] are being applied consistently, apart from the stories people tell us - if you're in front of one judge in one court you can get a different sentence to a different judge in a different court, and that's not right."

The plans have been slammed by the Opposition, with National MP Amy Adams saying that Little wants to let offenders back into the community.

"I think that actually the concern here is that Mr Little is asking judges to be more lenient and have more offenders walking around in the communities awaiting trial rather than in prison where that's warranted."

She also criticised the suggestion that judging is inconsistent, saying that the judiciary should be independent of the Government.

"To have a Govt minister indicate that he wants to set up a body to control and dictate the range of sentences that judges can put in place, I think is very, very concerning."