The offender, who cannot be identified because he was 17 at the time of the incident, faced a judge-alone trial in the ACT Supreme Court earlier this year.

He pleaded not guilty to two counts of rape and one count of committing an act of indecency without consent, following an incident in April 2018.

On Thursday, Justice David Mossop found the offender guilty of all three charges.

Justice Mossop's judgment says the victim, who was also 17, picked the offender up from a birthday party in Barton on the night in question.

The pair stopped in a car park and initially engaged in consensual activity, including kissing, in the back seat of her car.

The offender eventually asked the victim if he could give her oral sex. She said no, but he went to do it anyway and licked her thigh.

He subsequently penetrated the victim multiple times despite her telling him "no" and "stop". The victim eventually stopped protesting, turned her head and cried, and the attack finally stopped about 20 seconds later.

During the trial, the victim gave evidence that the offender had claimed at times that he could not hear her as she protested, and that he had held her down when she tried to push him away.

She said that when she started crying, the offender asked: "What, were you being serious?"

Following this, the victim said, the offender grabbed the car keys and spent about 40 minutes wanting to discuss what had happened. She said he was continually saying sorry, hitting himself, and asking whether she thought he had raped her.

The victim said the offender finally gave the keys back when she said she was not going to tell anyone. She then dropped the offender home and pulled over down the street, crying hysterically and hyperventilating.

She subsequently told friends, a friend's mother and her own parents what had happened.

There was then a period in which the victim's parents and the offender's parents spoke. The offender's mother is said to have expressed concerns about implications for her son's sporting contract if the victim went to police.

The victim did go to police, a little more than a week after the incident.

The offender gave evidence that he had drunk up to six beers at the birthday party.

He said the victim had agreed to have sexual intercourse, but it stopped after about 10 seconds because "it just got real awkward".

He denied withholding the car keys from the victim and said they had "a general chat" before she took him home.

Justice Mossop found the offender's claims "positively implausible".

"There was no adequate explanation of why matters 'got real awkward' or why a fully aroused 17-year-old boy ... would cease intercourse after 10 seconds with a young woman who, on his version, was fully consenting," the judge said.

Justice Mossop said he was not satisfied that the offender had deliberately held the victim down, but there was no doubt the three offences had occurred during the incident.

He said the offender had been reckless, at the very least, as to whether the victim consented.

The judge said it was not until the victim started crying that the offender's attitude changed, in a telling sign.

"I am satisfied beyond reasonable doubt that the accused then recognised that he had gone too far and had proceeded in circumstances where the [victim] had not consented to the sexual intercourse," Justice Mossop said.

"It is that which provides the reason, consistent with the [victim]'s evidence, for [the offender] wishing to discuss what had occurred with her, in the hope of reducing or eliminating any upset on her part and thereby protecting his own interests."

The offender will be sentenced at a later date.