The Maori Party and the Green Party are asking the Government why prosecution rates of young Maori appeared to be far higher than Pakeha for the same crimes.
Justice reform group JustSpeak released data which showed that the likelihood of a prosecution for a Maori person aged 10 to 16 was higher in every category of crime except one compared to a Caucasian.
The figures, compiled from New Zealand Police records, showed that 46 per cent of Maori youth apprehended for sexual assault were prosecuted, while 34 per cent of Pakeha faced prosecution. The only category in which Caucasians were more likely to be prosecuted was "miscellaneous offences".
Maori Party co-leader Tariana Turia challenged Police Minister Anne Tolley in the House to explain the discrepancy between Maori and Pakeha prosecutions yesterday.
Ms Turia said that disproportionate treatment of Maori in the justice system had first been identified 30 years ago, and questioned whether this bias was being addressed.
Mrs Tolley rejected accusations of institutional racism.
She said the data did not take into account many factors such as whether the crimes were committed by repeat offenders or whether evidence was available to help with a prosecution.
The minister cited the "Turning the Tide" plan, introduced last year, which aimed to reduce the proportion of first-time Maori youth and adult offenders by 10 per cent.
JustSpeak spokeswoman Lydia Nobbs noted that the data had some limitations, but she emphasised that it showed an overall trend that the Government should consider.
"We don't know the seriousness or details of each crime recorded, but a picture emerges with some surprises. To the Government we ask 'please explain'.
"When the Government releases its Youth Crime Action Plan later this year, JustSpeak will be looking closely for how the Government plans to address these sometimes surprising outcomes for young people."
The Green Party said it had written to new Race Relations Commissioner Susan Devoy to ask her to investigate the trend identified in the police data.