The Sydney University student was dancing with friends and kissing a man at the St John's College end-of-year formal.
The next thing the woman remembered was being in a leafy area with overhanging trees on the grounds of the college with the man, Jean Claude Perrottet on top of her, the NSW District Court heard.
Mr Perrottet, 20, the younger brother of NSW Treasurer Dominic Perrottet, is charged with three counts of having sexual intercourse with the 19-year-old woman without her consent in the early hours of October 18, 2015. Mr Perrottet has pleaded not guilty.
NSW District Court judge Stephen Norrish informed the jury panel of Mr Perrottet's politician brother before 12 members were selected.
"I bring that to your attention not to promote the accused, or say anything against him, but ... the name Perrottet may circulate in your mind," Judge Norrish said.
Crown prosecutor Fiona Gray told the jury she expected the woman to say she drank a cocktail and champagne throughout the evening.
"Some of [the woman's] friends noticed that she appeared to be affected by alcohol, she appeared to be drunk to them," Ms Gray said.
At that point late in the evening, Mr Perrottet – who was not a college resident or uni student – joined the woman and her friends on the dance floor, set up in an area known as the Lemon Grove.
The pair kissed while dancing, Ms Gray said.
"The next thing she recalls is being in an area with overhanging trees … and the accused on top of her."
Ms Gray said the jury would hear evidence that Mr Perrottet allegedly sexually assaulted her, and she told him: "Ow. I don't want to do that."
Despite the woman's continuing protests, Mr Perrottet allegedly sexually assaulted the woman twice more, Ms Gray said.
"She felt scared at that stage."
The court heard the pair went back to her college dorm room together, and when a concerned friend knocked on the door, Mr Perrottet allegedly said: "I'm not going to have sex with her, we are just talking."
The woman's male friends told Mr Perrottet to leave.
Defence barrister Alissa Moen said one of the alleged sex acts did not happen, and the other two were consensual.
"What is in dispute … is consent. The defence contention is that that was entirely consensual," Ms Moen said.
Ms Moen said Mr Perrottet was of good character, and unlikely to engage in the alleged acts.
She urged the jury to ignore any media publicity about the case.
"The general topic of sexual assault on campus or sexual assault in general has received a lot of media attention," Ms Moen said.
"But this case is unique to its own circumstances."
