Sam Ian Ballantyne, then 19, was at a house party with the 17-year-old in Canberra in 2012.

Both had been drinking and the victim decided to go to sleep in a lounge area in the early hours of the morning, as the party began to wind down. Ballantyne came into the room, lay behind her, pulled down her stockings and then sexually assaulted her.

The girl sat up, saw him and retreated to the kitchen, becoming hysterical and immediately telling the remaining revellers that she had been sexually assaulted.

Ballantyne, now 21, was sentenced for the crime on Tuesday and the ACT Supreme Court heard how his actions had devastated the victim and her family.

Her mother wrote that her daughter became incredibly afraid after the sexual assault and was too scared to leave their home.

She dropped out of school, constantly feared encountering Ballantyne and began to suffer terrible nightmares.

''The person who came home to me after you raped her had nightmares,'' the mother said. ''Every night, I hold my shrieking, sobbing, shaking daughter in my arms.''

The mother said her daughter could recover but would never regain her innocence.

''My daughter is not broken - she is strong, resilient and brave,'' she said. ''But she is permanently scarred.''

Ballantyne protested to the court as the mother's victim impact statement was read out, as he attempted to express his disagreement with some aspects of the mother's statement. That prompted Chief Justice Helen Murrell to tell barrister James Lawton that his client's protests were not helping.

''It wouldn't be unhelpful to display some remorse at this particular juncture,'' Chief Justice Murrell said.

The court heard Ballantyne had only a minor criminal history and the Crown conceded the offence was at the lower end of seriousness for offences of its type.

The offender was young, working and had good prospects of rehabilitation.

Mr Lawton urged the court to impose a periodic detention order combined with community service or a suspended sentence.

But the Crown argued Ballantyne had displayed a concerning lack of responsibility and remorse for the offence.

Chief Justice Murrell sentenced Ballantyne to 18 months' imprisonment, to be served by six months of weekend detention.

The rest of the sentence was suspended and Ballantyne will be put on a 12-month good behaviour order.