Michael Alan Gillard, 58, was convicted after a jury trial in the ACT Supreme Court last year.

He is appealing against his sentence and conviction.

The victim was a teenager when Gillard began abusing her in the 1990s, forcing her to have sex with him or perform sex acts on him.

The abuse took place at Gillard's home in Canberra, where the victim and her sister stayed when they came to visit their brother, who was then living in the territory.

During Gillard's trial the court was told he was a good friend of the girls' father and they looked upon him as an unofficial uncle or godfather.

In September last year he was found guilty of eight sexual offences.

He was acquitted of four sex charges. Gillard is also awaiting sentence for two sexual offences against the woman's younger sister.

During the trial the Crown had argued Gillard had a close relationship with the victim's family and abused his position of trust as the girl's unofficial godfather.

But yesterday the Court of Appeal heard the woman and her family had given evidence at trial that was wrong and undermined the Crown's case. Defence barrister Ken Archer said his client had not seen the family for years before he moved to Canberra and the sisters started coming to visit.

But the Crown argued that the relationship between the two was borne out by Gillard's interviews with police, in which he said he was her unofficial godfather and ''love[d] her dearly because I've known her since she was about three''.

Mr Archer also said the sentence of nine years and nine months handed down by Chief Justice Terence Higgins was excessive.

''There are few if any cases in the territory … involving such a long head sentence with such a significant non-parole period,'' he said.

Crown prosecutor Hament Dhanji SC said Gillard's offences were part of a course of events and the overall sentence reflected the totality of the crimes.