Michael Alan Gillard, 61, repeatedly abused the two girls in the 1990s, while he looked after them during school holidays at his Canberra home.

His crimes had a devastating impact on the girls' lives, and were described by his sentencing judge as a "gross breach of trust".

The older sister wrote of her pain in a victim impact statement, tendered during Gillard's sentencing late last year.

"He took away my childhood, my innocence and my freedom," she wrote.

ACT Supreme Court Justice John Burns sentenced Gillard to 10 years and nine months imprisonment, with a non-parole period of seven years, in December.

Justice Burns said Gillard had a history of denial and had repeatedly sought to downplay his own culpability

The sentencing came after years of appeals launched by Gillard.

He originally faced two trials in 2011, and was found guilty in both.

But Gillard successfully challenged some of the convictions from his first trial in the High Court, which ordered a retrial on those charges.

That retrial never took place because Gillard made a plea deal with prosecutors in October, allowing Justice Burns to proceed to sentence.

But now, in the latest twist to the protracted proceedings, Gillard has launched a bid to reduce the sentence.

He filed the appeal early last month.

Court documents lodged with the ACT Court of Appeal by his lawyer, Michael Kukulies-Smith, of Kamy Saeedi Law, argue the sentence was "manifestly excessive".

Gillard will argue the different sentences for his individual crimes were accumulated to an inappropriate degree.

The result, he will argue, was a "crushing" sentence. A crushing sentence is typically one that brings a sense of hopelessness to the offender and destroys any expectation they may have had of a useful life post-release.

The prisoner will also argue that the court did not give him discounts for two of his guilty pleas, despite it being required to do so by law.

The case will come before the ACT Court of Appeal for directions later this month.