A teenager who was accused of raping a fellow student from a Blue Mountains high school during a house party has had his convictions overturned on appeal. 

The girl's family is now planning a civil case against the boy, her father said after the decision was made in Penrith District Court today.

In August, the boy was handed a sentence of nine months in youth detention, with a non-parole period of two months.

He was found guilty of six counts of sexual assault and one count of choking over the 2019 incident.

Parramatta Children's Court previously heard the boy initially engaged in consensual sexual activity with the girl in a living room at the party when older guests had either left or gone to sleep.

The two, who were the same age and had been drinking, gave differing accounts in court about what then occurred on a mattress.

The girl gave evidence she lost consciousness and said "stop" multiple times.

During his evidence, the boy recalled being "cautious about consent" during the initial activity and denied there were times when the girl appeared unconscious.

The court heard of messages and phone calls in which the girl told friends she had sex at the party and was drunk, but about six months later told a friend it was not consensual.

She made a disclosure to a teacher that she was raped about one year after the incident and recalled feeling a need to "get it off my chest".

The boy had previously been acquitted of three charges over the same incident.

During the appeal, his lawyer submitted a magistrate's findings were "glaringly improbable" and contrary to compelling inferences.

These include the evolving warmth of their contact at the party, despite the fact the boy knew she didn't like him.

The lawyer argued the complainant feared the negative consequences of her parents finding out and of people spreading rumours.

They also highlighted the interactions with friends in which there was no mention of the activity being non-consensual.

Judge Huw Baker noted the matter was a case of "oath against oath" and the complainant's evidence was largely consistent and coherent.

He found the lies she later told adversely impacted upon her credibility, however, there was also a "degree of suspicion" about the boy's evidence.

"Overall I found his evidence concerning the sexual activity to be consistent and not implausible," the judge said.

"I'm not satisfied of the appellant's guilt for each of the charges beyond reasonable doubt".

After the decision, the girl's father said the experience left his daughter with severe Post Traumatic Stress Disorder, and the legal action "further compounded" the trauma.

The father also raised concerns the appeal judge was not aware of elements of the lead-up to the incident, including that the school had promised to have the male pupil spoken to due to his "harassment" of the girl.

He flagged a civil case against the boy, but said: "Our only concern is our daughter may not be well enough to take that legal action."