A synthetic drug addiction was behind a teenager's attack on a Christchurch nursing student

19-year-old Tane Faatafa faces just over two-years in prison - after pleading guilty to possessing a knife and robbing a student on Hagley Avenue late at night in April last year.

Faatafa wore a black face mask, held the young woman down and punched her in the face 10-times ... as she returned to her car after working a night-shift.

Judge Tom Gilbert says the teen has a history of drug use, but could still turn his life around if he makes positive choices.

The court heard how the victim now can't sleep without pills and is constantly on edge.