Northland Maori are twice as likely to go to jail than Pakeha when convicted of assault, sparking calls from Te Tai Tokerau MP Kelvin Davis for a Government inquiry into the "biased" criminal justice system.
But a leading criminologist says a "class bias" rather than prejudice against Maori is behind the figures.
Ministry of Justice figures reveal in Northland in the last 10 years, 22.2 per cent of Maori convicted of assault were imprisoned, compared to 10.6 per cent of Europeans - when both were found guilty of the same crime. Last year the situation was even worse, where 30.3 per cent of Maori convicted of assault were imprisoned, compared to 7.5 per cent of Europeans.
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.
But Labour's spokesperson for Maori development Mr Davis said there's an unconscious bias "right throughout the judicial system".
"We just can't keep locking up brown people at twice the rate of non-brown people," he said.
He said incarceration rates were one small part in the web of the "ugly" criminal justice system that disproportionately affected Maori.
"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."
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. Across all ethnic groups, community work was the most common sentence type for people convicted of assault, comprising of European (28.9 per cent), Maori (31 per cent) and Pacific (30.8 per cent) of cases.
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 many 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, Maori and communities. However, we are setting a clear direction of travel and focusing on initiatives that will make the biggest difference," Ms Adams said.
"While many of the drivers of offending lie outside of the justice sector, I am of the view there is more the justice sector can do to reduce Maori over-representation both as offenders and as victims."
Europeans convicted of assault paid a fine, reparation or restitution in 13.6 per cent of cases - a more common sentence than imprisonment. In comparison, convicted Maori paid a fine, reparation or restitution less than 6 per cent of the time.
In 2015, 7598 people were convicted of assault - 3928 Maori, 2505 Europeans, 894 Pacific People, and 271 people of other ethnicities.