The Green Party is calling for an investigation into why Maori are over-represented in criminal justice statistics.

Youth law advocacy group Just Speak says official statistics show Maori youth are disproportionately prosecuted for crimes compared to Pakeha.

The Greens have written to Race Relations Commissioner Dame Susan Devoy asking her to investigate the structural discrimination they say is revealed in the statistics.

In Parliament on Wednesday, Maori Party co-leader, Tariana Turia asked Police Minister Anne Tolley to acknowledge that institutional racism within the police contributes to the disparities shown in the numbers.

Ms Tolley strongly denies any bias in the police.