A 16-year-old boy has been taken to the Alfred Hospital in a stable condition with upper body injuries following the incident in Ashburton about 2pm.
According to police, the teen was approached by a male on High St near Ashburton Station.
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Investigators believe the male grabbed the boy’s bag then assaulted the teen with a sharp object.
The offender then fled north along Welfare Parade, then went east down a laneway, with the teen’s bag.
The offender is believed to be Caucasian in appearance, approximately 16-years-old and about 170cm tall with a slim build and long blond hair.
He was wearing a black top and grey tracksuit pants.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000Internet Explorer Channel Network