An ACT Supreme Court judgement published on Monday reveals that the offender, who cannot be named because of his age, was 16 at the time of the March 2019 incident.

He was in the same school year as his victim, who is four months his junior, and the pair lived in the same area of the ACT.

On the day in question, the girl walked to the boy's house after school and fell asleep on his bed.

She woke to feel the boy digitally penetrating her and saw that he had closed his bedroom door.

When she asked, "What the f--- are you doing?", the boy quickly moved his hand away.

The girl gathered her things and left, and a short time later the boy sent her messages on Instagram apologising for what he had done.

"I just want to say sorry I got carried away," the boy said in one message, which was accompanied by "cold sweat" and "worried face" emojis.

The boy pleaded guilty in May to one count of sexual intercourse with a person under 16, but told the author of a pre-sentence report that he "didn't do it" and that he had only pleaded guilty because his lawyer had advised him to.

His attitude later changed and he indicated in a second report that he accepted the allegations against him.

He also said he would like to write a letter saying sorry to the victim because he had recognised that his offending would have impacted on her "drastically".

The pre-sentence report author ultimately concluded, however, that the boy continued "to demonstrate minimal insight into the charge and the impacts it has had on the victim".

The boy was nevertheless assessed as having the capacity to comply with a good behaviour order.

In a statement to the court, the victim said she "no longer saw a future" for herself after what the boy did.

She described feeling betrayed after realising that someone who had been close to her could violate her in such a way.

The victim also said she sometimes stayed awake at night in an attempt to avoid experiencing flashbacks.

In sentencing the offender last week, Justice David Mossop said comparable cases showed that minors who pleaded guilty and showed some remorse could be given non-custodial sentences.

He described the boy as having "generally good" prospects for rehabilitation, given he had no criminal history, was on track to complete his schooling and was employed in multiple jobs.

The judge also said that because there was no evidence of how long the penetration had lasted before the victim woke up, he had to sentence the boy on the assumption it had been "relatively brief".

"So far as rehabilitation is concerned, the only area which requires some attention is in relation to his understanding of, and attitude towards, relationships with women and appropriate sexual conduct," Justice Mossop said.

"Those issues are ones in relation to which many young men need assistance but, so far as the offender is concerned, some intervention as part of his sentencing is appropriate."

The judge convicted the boy and sentenced him to a two-year good behaviour order for the "opportunistic" crime.

The conditions of the order include that the boy attend any programs, counselling or treatment - "particularly in relation to relationships and consent" - deemed appropriate by ACT Corrective Services.

Justice Mossop also found that the boy did not need to be placed on the child sex offender register because the victim was a very similar age and there was no evidence of an inappropriate sexual inclination towards children.