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George Waaka Hadfield, 23, was arrested by police after the violent flare-up in broad daylight on October 16 last year.
Hadfield appeared in the dock at Christchurch District Court on Wednesday afternoon to plead guilty to a charge of wounding with intent to cause grievous bodily harm.
He will be sentenced later in the year.
The court heard that Hadfield was with his partner at New World supermarket in St Martins when a man approached Hadfield in an aggressive manner, "making comments in support of Black Power".
They started yelling at each other, leading to a brief fight, before it broke up.
A woman and her partner started walking away towards Wilson Rd.
Hadfield followed them and more words were exchanged.
The woman and Hadfield's partner got into another fight which ended up on the ground.
Hadfield then "came in with a knife" and stabbed the victim in the back between her ribs, also slicing her right ear.
Hadfield and his partner then fled in a car.
The court heard that the victim was stabbed in the back, between the ninth and 10th rib, causing hemothorax – a collection of blood in the space between the chest wall and the lung. Hospital staff also recorded an internal liver laceration "due to the trauma".
The wound to her right ear needed stitches.
When police caught up with Hadfield, he said he did not stab the victim "and that it was a Mongrel Mob member who had passed him in a motor vehicle as he walked home".
Defence counsel Andrew McKenzie said a cultural report was being prepared before sentencing.
Judge Tony Zohrab gave Hadfield a warning under the three strikes law and remanded him on electronically-monitored bail. A sentencing date is yet to be confirmed.