A witness has told a Christchurch court she saw a local man punch his son and flick his ear.

Fifty-year-old musician Jimmy Mason is on trial on three charges of assaulting his two sons, then aged two and four, in December 2007.

Belinda Payne told the court she saw Mason at the Bridge of Remembrance where the two boys were riding their bikes.

She says he was yelling and telling off the boys. Ms Payne says Mason lifted up the bikes while the boys were still on them and forced them down onto the ground.

She says Mason then flicked the ear of the older boy and punched him in the face.

Mason's defence lawyer has told the court his client used reasonable force.