James Nicholas Williams, 21, pleaded guilty in a South Australian court to unlawful sexual intercourse with a person under 17.

He was jailed on April 9, 2024, for one year, two months and eight days, of which he must serve six months before being eligible for parole.

The court heard Williams, then aged 20, travelled with members of his football club to Adelaide during September, 2022.

The group stayed at a backpackers hostel.

On the night of September 16 the group went to the Hindley Street Music Hall to see an artist perform.

Williams, who had been drinking since 7am, was standing in the mosh pit area next to the 15-year-old victim, who attended the concert with her mother.

He brushed against her arm, they talked and she told him she was 15.

Williams told the victim she looked older than that, did not respond when she asked him his age, and then added her on social media application Snapchat.

The court heard the victim was dancing and filming the concert when Williams placed his hands around her waist, started kissing her neck and they both kissed on the mouth.

Williams then placed his hands under the victim's skirt and despite her repeatedly asking him to stop, he sexually assaulted her.

When the victim said "no, stop", Williams replied "no, it's fine".

When the girl's mother saw the man's hand up her daughter's skirt she grabbed his hand.

But he returned his hand back up her skirt and glared at the mother.

Security was alerted and Williams was kicked out of the venue.

He was arrested on Hindley Street in the early hours the following morning and found to have alcohol and illicit substances in his system.

Judge Michelle Sutcliffe said while Williams admitted the offending, he claimed he was not aware the victim had not consented.

"I do not accept that," she said.

"You disregarded her refusal by telling her it did not matter."

The judge said the offending had a "catastrophic impact" on the victim's well-being and impacted the persons close to her.

"She described how the trauma of having her choices removed has affected every aspect of what remains of her childhood as she learns to deal with post-traumatic stress disorder, dealing with panic attacks and being in environments that trigger her distress and fear," she said.

"The victim has lost friends, who do not understand her needs. She has missed school and resigned from her employment."

Williams' lawyer submitted his conduct was "totally out of character and explained by his intoxication" which occurred in a context of peer pressure where the man was with older members of his football club, who also gave him drugs.

The judge said she accepted Williams' intoxication impaired his judgement and that he was "deeply ashamed and genuinely remorseful".

But she said attending a music concert was often a right of passage for young people.

"Teenage girls are entitled to feel safe, to be safe and have an enjoyable time, free from the worry of being sexually assaulted at these events," she said.

Williams played football for Tyrendarra Football and Netball Club.