The 70-year-old man, who cannot be named, was resentenced on Friday to four years and 10 months behind bars by the ACT Court of Appeal, which added 20 months to his original jail term.

Three judges also set a new non-parole period of two-and-a-half years, bumping the minimum period he must spent in prison up by 10 months.

The former Christian minister previously admitted he had sexually abused his son between 1997 and 2002, pleading guilty to three charges that included one of maintaining a sexual relationship with a child.

The other two offences were of committing an indecent act on a child.

Justice John Burns said in sentencing that the victim was aged between 11 and 15 when the offender, who presented himself as a man of God, practised "the very antithesis" of what he preached.

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"You are not to be sentenced for hypocrisy, but it is relevant to note that the difference between the standards and values you advocated as a pastor and those which you applied in your own home were apt to confuse and warp your son's appreciation of the distinction between right and wrong," Justice Burns told the man in the ACT Supreme Court last July.

The judge added that the child sex offender had attempted to minimise his responsibility for his crimes, having tried to lay some of the blame on the son he used to "summon" to his bed in suburban Tuggeranong on Saturday mornings.

When sentencing the man to three years and two months in jail, with a non-parole period of 20 months, Justice Burns accepted the offender had shown some remorse and took into account the fact prison would be particularly difficult for someone his age.

Prosecutors challenged the severity of that sentence in the Court of Appeal last month on the basis it was "manifestly inadequate".

They said allowing it to stand would "undermine public confidence in the administration of justice in the ACT".

Justice Michael Elkaim, acting Justice Verity McWilliam and acting Justice Stephen Walmsley agreed with the inadequacy argument in a judgment delivered on Friday.

"The conduct was clearly egregious, occurring over a protracted period, in the victim's home, and at the hands of his own father," the three appeal judges said.

"The victim suffered greatly from his father's betrayal of him."

In light of these and other factors, the trio found the sentence imposed by Justice Burns did not "reflect the gravity of the offending and the devastating harm to the victim, which is lifelong".

Under the terms of the new sentence, the offender will become eligible for parole in January 2024.

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