Joseph Stephen Rosenburg, 51, has pleaded not guilty to three charges of sexual intercourse without consent and three counts of indecent assault, and is on trial in the Downing Centre District Court.

The 18-year-old woman, who cannot be identified, was out with friends at Q-Bar on Oxford Street, Darlinghurst, where she consumed "a large amount of alcohol", and got into a taxi with an unknown man late on August 29, 2013, crown prosecutor Craig Patrick said in his opening address on Tuesday.

The taxi stopped and drove away as the woman got out alone and vomited, before she lay down near a building across the road from Hyde Park, Mr Patrick said.

He told the jury they will hear evidence that Mr Rosenburg then allegedly approached the woman and put his hand up her dress.

Mr Patrick said Mr Rosenburg then allegedly followed the woman into the park, in Sydney's CBD, and forced her to have oral sex and anal sex, knowing she did not consent.

During the alleged attack, the woman asked Mr Rosenburg, "Are you going to kill me?" and he replied, "No, I'm not going to kill you if you go with it", the court heard.

Mr Patrick said the woman also told Mr Rosenburg "Don't hurt me, don't hurt me", to which it's alleged the accused replied with words to the effect of "I'm not going to hurt you".

The woman sent several text messages to friends during the alleged assault, including "I'm being tapes [sic]. Help."

The prosecutor said the woman ran away and spoke to a friend on the phone, telling her: "He's hurt me, he's hurt me, he raped me, he raped me."

The woman's friend found her sitting outside St James railway station, and took her to Liverpool Hospital.

DNA found on the woman's underwear and her body matched the profile of Mr Rosenburg, Mr Patrick said.

Defence barrister Ken Averre said there would be no dispute that the DNA belonged to Mr Rosenburg, but that consent would be an issue.

"What will be in dispute is how that came to be, so exactly what occurred in the early hours of August 30," Mr Averre said.

Mr Averre said that there was some consensual touching and other sex acts between the woman and Mr Rosenburg, but the act of anal sex was in dispute.

The trial continues before Judge Stephen Hanley.