The chief executive of a women's health charity says she was indecently assaulted by a former Knox Grammar School headmaster.

Lucy Perry has told a royal commission hearing into the elite Sydney north shore boy's school that Ian Paterson groped her in 1989 when she was part of musical being produced by her school Roseville and Knox Grammar.

Ms Perry who is chief executive of Hamlin Fistula Ethiopia (Australia), said the assault happened in full view of students at the Knox school hall, and the boys who saw it "roared with approval".

She reported the assault to NSW police in 2009 when numerous allegations about widespread sexual abuse at the school were coming out.

Ms Perry said at one of the rehearsals she was backstage talking to some of the boys at a production of Guys and Dolls when Dr Paterson pointed to her and one of the boys and said "you and you out".

Despite this she turned up at the next rehearsal and Dr Paterson who was the produced singled her out.

As she walked to the edge of the stage "he placed his hand on my backside. I would describe it as between a pat and a grope".

"He then slid his hand right down to cup my buttocks and (I) felt him touch my genitals outside of my clothing".

She said he had a creepy look on his face.

Ms Perry, 41, said all the boys were sitting on the floor and the assault happened in their plain sight.

"I was humiliated in front of the boys and I heard the boys cheering.

"I was disgusted with Paterson's behaviour and thought he was demonstrating to the Knox boys that it was perfectly acceptable, even admirable to be disrespectful to girls."

She said she was not interested in pressing charges against Dr Paterson but in 2009 decided to report it to police.

"I wanted to let police know that I was happy to provide a statement if it was going to be useful to demonstrate the culture of disrespect engendered by the school and to highlight the environment and attitudes fostered by the headmaster."

She also said she came forward now because she wanted to help a commission committed to bringing about institutional change.

When asked by Jim Harrowell, a solicitor representing Dr Paterson, if he was just positioning her on the stage she said: "I would suggest to you Mr Harrowell that positioning girls by their bottoms is not the best way to do that".

She received a standing ovation from Knox victims and their supporters as she left the hearing.