A university lecturer who sexually assaulted a 12-year-old girl who was a friend of his daughter while at a birthday sleepover has been sentenced to four years in jail.

The academic, 53, was last month found guilty in a County Court trial of molesting the girl while she stayed at his home one night in March 2011.

A jury earlier this year was unable to reach a verdict on some of the charges, which prompted a retrial.

The court heard on Wednesday the girl woke during her stay to find her hand being held by the lecturer, before he sexually assaulted her.

The girl told the man to "stop it" and threatened to go to the police and kicked him off her, after which he left the bedroom, the court heard.

The sentencing judge said the girl had been fond of the man - as the father of her best friend - as she had spent a lot of time at his home while her own father was recovering from a stroke.

The judge said the academic had exploited the girl's vulnerability and that of her family, which had been hit hard emotionally and financially by the effects of her father's stroke.

"You breached not only the trust of the complainant but also the trust her parents had placed in you allowing you to care for their daughter when she was at your home," the judge said.

The academic was found guilty of three charges of committing an indecent act with a child under 16 and one count of sexually penetrating a child under 16.

He was discharged on two other charges related to an alleged incident in May 2011 after the jury could not reach a verdict.

The judge said the lecturer was a well-respected member of his community and university and had been well regarded by his colleagues, friends and students.

The judge accepted the man's reputation was now in tatters and that his own family had been subjected to abuse and his children had been bullied at school.

But the judge said the man's "appalling behaviour" had caused his victim and her family great distress, and that the judge had to denounce the breach of trust the academic had committed.

The lecturer must spend a minimum two years in jail before he is eligible for parole.