A Sydney University student told a friend "she was with someone and she didn't like it" after her alleged rape at an end-of-year college formal, a jury has heard.

Jean Claude Perrottet, the 20-year-old brother of NSW Treasurer Dominic Perrottet, is on trial in the NSW District Court after pleading not guilty to three counts of sexual intercourse without consent.

A friend of the alleged victim on Thursday said she was called to the young woman's dormitory room some time after they separated at a St John's College formal in October 2015.

"She said that she was with someone and she didn't like it and that she didn't want to," the friend said.

"I said did she know who it was, and she said she didn't know, and then I said that it was rape."

The court previously heard how the woman found herself entangled with a man she had met earlier on the night of the ball, but she had no idea how she got there. 

The woman told the court she knew that she had been kissing the young man on the dance floor, but after six drinks on an empty stomach she could not remember the journey across the quadrangle of St Johns College to the area known as the tree canopy.

She could not remember the two of them kissing as they stood under the tree canopy, nor how they ended up on the ground.

But giving evidence in the NSW District Court on Wednesday, she was sure she told him to stop.

Mr Perrottet was not a resident of the college and the pair had not met before the night of the 2015 college ball, where he has been accused of raping the woman.

He has pleaded not guilty to three counts of sexual assault without consent.

The trial continues.

AAP, Fairfax Media