Police say a teenage girl ran for help after escaping a man who allegedly sexually assaulted her near other festival-goers at Groovin the Moo in Bunbury, one of Australia's most popular music events.
A police spokeswoman said the girl did not know the man who allegedly assaulted her early in the evening on May 6.
Detectives were called in soon afterwards, she said.
"A teenage girl was sexually assaulted by an unknown man in the area referred to as the Cattleyard stage at the festival, sometime between 6.30pm and 7pm," she said.
"The victim managed to get away from the man and report the incident to police."
Police urged festival-goers to come forward with any information.
"Numerous people were in the area at the time of the incident," the spokeswoman said.
"Detectives wish to speak to anyone who saw a male acting suspiciously in the area during the above time."
Organisers working with police
Groovin the Moo is held in six regional centres around Australia and attracts thousands of people.
A spokeswoman said the festival's organisers were aware of the reports of a sexual assault in Bunbury.
"As always, we will work with police on any event incidents," she said.
She said organisers had measures in place to protect attendees and that there was CCTV at the entry to the event.
"GTM Bunbury had a large police, security and medical operation," she said.
"In addition to this we employed a large crowd care team through Red Frogs.
"We also had youth liaison officers working for the duration of the event who were on hand for patrons as friendly faces, providing them with assistance."