A Sydney judge has found Luke Andrew Lazarus not guilty of the sexual assault of an 18-year-old woman at the back of his father's Kings Cross nightclub in May 2013.

Mr Lazarus, was convicted and jailed in 2015, but in February last year the Court of Criminal Appeal found he was entitled to a retrial, after the judge misdirected the jury.

He served 11 months of a maximum five-year sentence.

In her ruling on Thursday, Judge Robyn Tupman said the crown had not proved beyond reasonable doubt that Mr Lazarus did not think the woman was consenting.

Judge Tupman said while the woman in her own mind did not consent, Mr Lazarus had a "genuine and honest belief" she was consenting.

"I accept she did not in her own mind attempt to convey consent to sexual intercourse," she said.

"[I] accept evidence the accused had a genuine and honest belief the complainant was consenting to sexual intercourse."

Judge Tupman said the encounter was quick and unromantic, and may not have happened if they were sober.

Lazarus had 'genuine belief' woman consented: Judge

The nightclub where the pair met was closed in 2015. ( Supplied )

Mr Lazarus had met the woman at the Soho nightclub that was owned by his father.

Evidence was previously given that he had walked up to her and told her that he was a part-owner in the venue, and showed her a business card.

He then took her to the alleyway behind the nightclub, where they kissed, a court heard.

The woman gave evidence that she had wanted to return to her friend, and that he said, "no, stay", before they eventually had sex.

The court had also heard that Mr Lazarus said in an authoritative tone "put your f***ing hands on the wall, get on the floor and arch your back".

"No doubt she was starting to feel she was in a difficult situation," Judge Tupman said in her judgment.

The woman said she later met with a friend, who told the court she found the complainant crying hysterically and describing herself as being in pain.

The next afternoon the woman was taken to the police station by her sister, and was forensically examined at hospital.

Judge Tupman said the complaint to police was made at a time and in a manner that would suggest it was reliable.

In the courtroom, Mr Lazarus was supported by a large group of friends and flanked on either side by his parents.

His mother broke down crying and embraced her son when he was acquitted.

The complainant, sitting only four seats away from Mr Lazarus, sat silently crying as Judge Tupman read her verdict, and ran out of the room immediately once he was officially acquitted.