A former petty officer in the navy who had a sexual relationship with a teenage girl in his care will spend the next year behind bars and another 18 months in weekend jail.
Justice Hilary Penfold on Wednesday sentenced the 39-year-old man, who cannot be named for legal reasons, to five years.
After serving stints in full-time custody and weekend jail, the remainder of his sentence will be suspended on entering a three-year good-behaviour order.
''It is a tragedy for all concerned that [the defendant] allowed himself to take advantage of the complainant's need for a father figure by acquiring, not a daughter, but an immature sexual partner,'' the judge said in her sentencing remarks.
The defendant has held communications roles with private contractors in the military sector and formerly served in the Royal Australian Navy.
The relationship between he and the 15-year-old victim began in 2010 when the girl came to stay with him and his wife, the victim's cousin.
The ACT Supreme court heard the child was in a vulnerable emotional state at the time, later exacerbated by a fight with her cousin.
The crimes came to light in October 2010 after the defendant's wife discovered the pair in bed and the girl broke off their relationship, saying she wanted the sex to end.
The victim confided in schoolmates, and police became involved.
The defendant eventually pleaded guilty to 11 counts of having sexual intercourse with a person younger than 16, charges spanning five months.
The court heard the man accepted what he did was ''illegal, serious and morally wrong''.
And the defendant admitted he was aware what he was doing was wrong while the sexual relationship was ongoing, but was concerned about what might happen if he ended it.
The girl later told police and the court she fully consented to the relationship at the time and thought she was in love with the accused. She said she was looking for a father figure because her dad was never around.
''I loved that we could talk, I just didn't like the sex that could happen, but I kind of saw it as a condition of our relationship,'' she said.
A person younger than 16 cannot legally consent to have sex.
A psychiatrist said the defendant was likely to have been in the grips of a depressive episode at the time of the offending.
The judge said while there was no specific evidence the girl suffered adverse consequences because of the crimes, there was a statutory presumption such offences caused harm to the victims.
Justice Penfold took into account the man's remorse, previously ''impeccable'' character, the ongoing support of his wife and the financial impact of the legal proceedings.
The court heard the man grew up in an environment where he and his siblings were sexualised at an early age through exposure to nudity, sex and ''swinging'' behaviour.
She also accepted he was more a ''foolish middle-aged man who thought he was in love'' than a ''cynical, conniving seducer''.
But the judge said the crimes were serious, he breached a trust and the victim's interest in him was born out of a need for emotional comfort rather than romantic desire.