An ACT Supreme Court jury found Michael Alan Gillard, 57, guilty of eight sexual offences against the then-teenage girl between 1995 and 2000.

But the jury cleared the man of another four charges of committing an act of indecency on a child aged under 16.

The court had previously heard Gillard was a family friend who had been considered an ''unofficial godfather'' to the victim and her siblings.

Some of the sex offences occurred when the victim and her sister stayed with him in Canberra during school holidays. Gillard forced the victim to perform oral sex on him in the presence of her younger sister in 1999.

He was found guilty of possessing child pornography after taking nude photos of the then-teenager with her legs spread.

Gillard was also found guilty of three acts of indecency on the victim between 1995 and 1997, when she was aged 14-16.

The man's defence barrister had argued any sexual activity between the pair only took place after the victim turned 16 and she consented to the acts.

It was argued all but one of the charges were a matter of ''one person's word against another'' and the victim had been a mature, intelligent young woman who could have refused to take part in a sexual relationship.

But the Crown had argued that even if the victim appeared to submit to the sex acts, Gillard had abused his position of trust as a family friend to develop a sexual relationship with her.

The court was told that even if the dates of the offences couldn't be pinpointed, that didn't mean the abuse did not take place.

Chief Justice Terence Higgins thanked the jury for their efforts in the face of a complex case with numerous charges.

Chief Justice Higgins continued Gillard's bail with extra conditions, placing the man under supervision from Corrective Services and ordering him to remain in Canberra and report to police twice a week.

He also ordered a pre-sentence report and a sex offenders' report.

Gillard returns to court in December for sentencing.