A Canberra man who molested his nieces when they were as young as three wrote them a letter 10 years later telling them God had forgiven him, a court has heard.

Key points: The man abused the girls during the 1970s and 80s

The man abused the girls during the 1970s and 80s He later wrote a letter saying he had been forgiven by God

He later wrote a letter saying he had been forgiven by God He was jailed for 10 years, and will not be eligible for parole until 2024

On Friday, the 70-year-old man was sentenced in the ACT Supreme Court to 10 years' jail for six offences, which ranged from carnal knowledge to indecent assault.

The court heard several of the offences were committed when the man was putting the girls to bed when he was babysitting them between 1979 and the mid-1980s.

The court heard the man, who cannot be named to protect the identity of his victims, had been a decorated Vietnam veteran, and had been supported throughout the trial by at least one sibling who told the court she had been surprised to hear of the allegations.

A pre-sentence report author noted the man had appeared to live a "positive" life, and never been in trouble with the law.

But they also said it was significant that the man did not appear to take responsibility for the offences and believed he had been the victim of vexatious accusations.

The court heard family members first confronted him about the crimes in the 1990s, but it was not until 2014 that he was informed police were formally investigating.

Justice Chrissa Loukas-Karlsson noted that prosecutors conceded the long delay between his offending and the charges should have been taken into account, but also considered some mitigating issues.

"The prosecution submitted that there is no evidence that he suffered suspense or uncertainty as to whether the authorities were going to take action," she said.

"The prosecution also submitted that from the letters he wrote to the victims in the 1990s it appears that he is at peace with his offending resulting from his offending due to forgiveness from God."

Justice Loukas Karlson also mentioned the devastating effect the events had on the victim's lives, addressing the man directly.

"Young children are unable to protect themselves," she said.

"Your young nieces were unable to protect themselves.

"This was a gross breach of trust."

The man was jailed for 10 years, with a non-parole period of six years, and will not be eligible to seek release until 2024.