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.

In a victim impact statement, the court heard how the woman’s partner heard the whole attack on the phone.

They also heard that the former student is constantly reminded of the assault and suffers from flashbacks and panic attacks. 

She now can't sleep without pills and is constantly on edge.

Judge Tom Gilbert says what Faatafa did only took a few seconds but made a lasting impact.

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