The 51-year-old man, who cannot be named, was sentenced to 6½ years' prison in September for two offences against his stepdaughter, who was under the age of 10.

He molested the girl while she was in his care, something acting Justice John Nield described in the ACT Supreme Court last year as a "breach of trust".

The man made claims that he was sexually abused by members of the Salvation Army during his childhood, which he said left him suffering anxiety and depression.

The claims have never been confirmed.

On Monday, the man appealed against his sentence in the ACT Court of Appeal.

His barrister James Sabharwal argued the sentencing judge had not properly explained how he had discounted the sentence to take into account the 20-year delay between the offences and the eventual prosecution.

The crimes against the girl occurred between 1989 and 1991, and she soon told her mother and was seen by doctors at the Canberra Hospital.

For unknown reasons, nothing was done in 1991 to prosecute the man.

Eventually, in August 2011, police issued warrants for his arrest.

That was something described as "unfortunate" by Justice Nield, who was required to take the "long and unexplained" delay into account as a discount in the man's sentence.

Mr Sabharwal argued on Monday that the judge had not properly quantified the amount of discount given to the offender for the delay.

He said the 6½-year sentence did not appear to have included any significant discount.

The court heard the High Court has discouraged judges from spelling out the arithmetic of each discount they make, instead encouraging them to employ an "instinctive synthesis" to arrive at the appropriate sentence.

In responding to the appeal, Director of Public Prosecutions Jon White said it was an "absurd proposition" that a sentencing judge should state the exact level of discount given for each and every factor in the case.

He said the assertion that the sentence was manifestly excessive was not supported.

The court reserved its decision.