The girl had to take off her underwear and lift up a pantyliner for the officer to inspect.
"She told me to squat on the ground. She then squatted down and looked underneath me."
She said she felt she could no longer trust police after the experience.
The Counsel Assisting the Commission noted that police regulations stated that a parent, guardian or support person must be present whenever a child between 10 and 18 is strip-searched.
She said the police did not have "untrammelled" right to do strip searches.
Of the 143 strip searches performed at the festival in 2018, police found drugs in only found 8.4 per cent of cases.
The public inquiry comes just days after a NSW coroner's draft recommendations were leaked calling for pill testing and for police to stop using sniffer dogs at festivals.
The inquiry is set down for four days.