Whanganui Maori are almost twice as likely to go to jail than Europeans when convicted of assault, sparking calls from an MP for a Government inquiry into the "biased" criminal justice system.
Ministry of Justice figures reveal in the Whanganui/Taranaki court area last year, 25.4 per cent of Maori convicted of assault were imprisoned, compared to 13.1 per cent of Europeans - when both were found guilty of the same crime.
Justice Minister Amy Adams said Maori have been over-represented in the justice system "for many years", and the sector was working to reduce this, but change would take time.
Former Te Tai Hauāuru MP and Government minister Dame Tariana Turia said Maori had fought "institutional racism" for years.
"It's a big worry for most of us, particularly those of us who have had kids go to prison because their lives change forever, and so do the lives of your family. It's a very significant issue," Dame Turia said.
"I haven't met anybody who's gone to prison who has come out a much better person for it."
Nationwide, since data was available from 1980, the highest rate of imprisonment for Europeans convicted of assault - 14.7 per cent - has not eclipsed the lowest imprisonment rate for Maori - 16.3 per cent - in any year.
Dame Turia said a historically informed approach to changes in justice system was needed to reduce disproportionate offence rates.
"We just look at the act, and we think the person has a propensity towards violence. In reality, there's an intergenerational issue here that's happened right back from colonisation actually, and everyone hates you for saying it, but it's the truth."
Te Tai Tokerau MP and Labour spokesperson for Maori development Kelvin Davis said there's an unconscious bias "right throughout the judicial system".
"There definitely needs to be an inquiry of sorts as to why Maori are being disproportionately sent off to prison and non-Maori are given fines."
University of Canterbury professor of sociology and criminologist Greg Newbold said the figures weren't surprising, but didn't necessarily indicate an ethnic bias in courts.
He said judges' sentencing decisions were informed by a number of factors, including prior criminal history, likelihood of reoffending, gang or organised crime connections and employment status.
"If you controlled for all those factors... I think you would find the courts were not biased against Maori.
"I don't think the courts are racially biased, I think they've got a strong class bias."
He said the Government should conduct research into the issue.
"The research needs to be done... They need to find out whether in fact these figures indicate a true bias or a false bias."
Justice Minister Amy Adams said reducing Maori over-representation in the sector was a "priority", and Government agencies were working on a comprehensive strategy to address it.
"Change will take time - there is no silver bullet and it will require effort from multiple government services, Māori and communities. However, we are setting a clear direction of travel and focusing on initiatives that will make the biggest difference," Ms Adams said.
In 2015, 7598 people were convicted of assault -3928 Maori, 2505 Europeans, 894 Pacific People, and 271 people of other ethnicities.