The 51-year-old, who cannot be named to protect the identity of the victim, pleaded guilty in the ACT Supreme Court to committing an act of indecency on a child under 10 and incest.

The court heard the man and his family had been living in Melba between 1989 and early 1991.

During that period, he was lying on the lounge with his step-daughter, then aged between eight or nine years of age, when he rubbed himself against her and touched her inappropriately.

In 1991, the family moved to a house in Holt where he forced the girl to perform oral sex on him.

The victim told her mother soon after and he admitted to the allegations when confronted.

He subsequently fled to Western Australia to live with his parents.

But 20-years elapsed before the man was extradited to the ACT to face charges.

The court heard the man was raised in the Salvation Army and claimed to have been molested by members of the congregation as a child.

The man also claimed to have no memory of the offending.

Acting Justice John Nield said the law forced him to impose a more lenient sentence because of the delay.

"It is clear that the offender regarded his step-daughter as his sexual plaything," Justice Nield said.

"[But] the offender is entitled to have the long and unexplained delay taken into account in his favour in the determination of an appropriate sentence for each offence.

"I suspect that the offender lived with the fear of being arrested."

The judge jailed the man for six-and-a-half years, backdated to take into account time spent in custody.

He will be eligible for parole April 2017.