It's 1.30am the day after St John's College end-of-year ball, and a young woman is telling detectives about being allegedly sexually assaulted in the car park.

"I remember being on the ground and lying there and saying, 'Stop it, get off me, I don't want to do that'," the 19-year-old woman told police at RPA Hospital soon after the alleged assault in the early hours of October 18, 2015.

"He kept trying to have sex with me and I was like: 'That really hurts. Stop it.'"

A recording of the woman's first interview with police was played to the jury at the trial of Jean Claude Perrottet​, the younger brother of NSW Treasurer Dominic Perrottet​, on Tuesday morning.

The 20-year-old has pleaded not guilty to three counts of having sexual intercourse without consent.

Speaking in a croaky, emotional voice, the woman told officers she was on the dance floor kissing a man "with a French name", then remembered ending up in the car park with him.

"He was having sex with me and I kept telling him to stop," the woman said.

Police asked her how much she had to drink, and she remembered having about six drinks.

"I knew I was drinking and trying to be drunk, but I didn't mean to get that drunk," she said, adding she hadn't eaten since breakfast.

The woman said she "snapped out" of it when she was in the car park.

"I was like, 'I'm not OK with that'. I do remember clearly saying to him: 'No. Stop.'"

Police noticed the woman, who was a student at the University of Sydney and a college resident, had dirty feet.

"Was that just from being on the ground?" a detective asked.

"Yes."

The jury also saw CCTV footage of the woman walking from the grounds of the college to her dorm room, with Mr Perrottet following closely behind, after the alleged assault.

"I went to my room, I think I thought there would be people in that corridor … but there wasn't," the woman told court, via audio-visual link.

The woman said Mr Perrottet "loudly" shut the door behind them when they reached her room.

"I think I was crying by that point. I don't think I said anything, I don't remember if he said anything."

The woman's friends eventually came to her room and told Mr Perrottet to leave.

She said she didn't initially speak to those around her, but one friend noticed some blood on her leg.

"[The friend] said, 'You've been raped'."

The woman said she told everyone in the room she wanted to go to bed.

"I felt quite claustrophobic, like there were lots and lots of people in my room and I guess I just felt really sore and upset."

Her friends urged her to tell police and go to the hospital.

Under cross-examination from defence barrister, Alissa Moen, the woman agreed it was dark during the alleged assault.

"I couldn't see clearly because it was dark," she said.

The trial continues before Judge Stephen Norrish.