Michael Alan Gillard, 59, was found guilty in two separate trials of abusing the sisters, who stayed at his home when visiting their brother in the ACT in the 1990s.

Gillard was friends with the girls' family, and they considered him as an uncle or godfather-type figure.

An ACT Supreme Court jury found him guilty of crimes against the older sister in September 2011, and he was later sentenced to nine years and nine months jail.

Lawyers appealed against the conviction, arguing the jury's findings were unsafe and unsatisfactory, that their client was prejudiced by changes made to the indictment after prosecutors had closed their case, and that the judge misdirected the jury.

They also challenged the nine year sentence, saying it was manifestly excessive and was calculated inappropriately.

But the ACT Court of Appeal dismissed their arguments and threw out the appeal in April this year.

Gillard had also been found guilty of abusing the younger sister, but the Supreme Court held off sentencing him for those crimes until the appeal relating to the other victim was resolved.

The matter has now come back before the ACT Supreme Court and, on Friday, defence lawyer Michael Kukulies-Smith revealed special leave was being sought to appeal the conviction in the High Court of Australia.

Mr Kukulies-Smith asked that the court wait until it was clear whether the High Court appeal would be allowed, before beginning sentencing hearings for the crimes.

But the Crown argued that Gillard should be sentenced as soon as possible for the abuse of the younger sister.

Prosecutors said the victim was distraught and distressed, and wanted the sentencing finished as soon as possible.

Justice John Burns agreed there was a question of giving closure to the victim.

But he said it was necessary to wait for the outcome of the High Court proceedings, which could ultimately change how Gillard is sentenced.

He adjourned the matter until mid-September.

Gillard remains in custody.