The alleged assault of an Aboriginal youth by north coast police in 2011, and the alleged cover-up that followed, had the "real capacity to seriously undermine public confidence in the New South Wales Police Force", the Police Integrity Commission (PIC) has heard.

The Commission has begun public hearings into the January 14 incident, in which Corey Barker, now 24, was arrested on Tamar Street, Ballina, and taken to the local police station.

The police officers involved claimed that when they attempted to move Mr Barker to a different cell he assaulted one of them - Senior Constable David Hill - punching him in the face and throwing his arms around.

However, when damaged CCTV footage was subsequently obtained by the prosecutor in Mr Barker's subsequent assault hearing, it appeared to show that there was no assault.