In the ACT Supreme Court on Friday afternoon, Acting Justice Verity McWilliam sentenced the offender, who cannot be named, to 23 months behind bars.

The man drew a sharp breath upon hearing the sentence, then hugged a support person before being taken into custody to start serving his non-parole period of 18 months.

He had previously pleaded guilty to one count of committing an act of indecency on a child.

Agreed facts show the man was living in Canberra's north with his then-partner, a child of their own and his six-year-old stepdaughter when the offence occurred in the latter's bedroom in April.

While he was supposed to be looking after the six-year-old, who called him "daddy", the man took off the girl's pants and underwear.

The 40-year-old man then touched the girl's vagina for about 20 seconds.

That night, the victim told her mother: "Mummy, I want to talk to you about daddy tonight after I go to bed. Um, he touched my 'giney' today."

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When the girl's mother confronted the offender with the allegation, he admitted touching his stepdaughter inappropriately.

However, he denied doing it on purpose and said it had happened while he and the girl were "mucking around".

The girl's mother booted the offender out of their home and called the police almost immediately.

When officers spoke to the man the next day, he told them the girl had refused to stop using her iPad when "screen time" was over.

He said he resorted to "roughhousing", and touched the six-year-old's vagina for about 20 seconds to get a reaction from her.

Acting Justice McWilliam rejected this claim on Friday, saying there was "simply no context" in which "roughhousing" would create a need to remove the girl's pants and underwear.

"The only reasonable inference for the court to draw from the offender's conduct is that it was done for sexual gratification," she said.

The judge went on to describe the offender's attitude to his crime as "problematic".

This was because he seemed only to understand that he had inflicted physical pain on the girl, with no appreciation for any of the other impacts of the incident.

These ramifications included that the victim, who once loved and trusted the offender, was now scared of being alone with any man.

Acting Justice McWilliam also referred to the impact on the girl's mother, who previously told the court in a statement that she would never forgive herself for bringing the offender into her family.

"She has cried many tears and had many sleepless nights," the judge said of this woman.

The offender's lawyer, Benjamin Rutzou, asked earlier this month for the man to be given an intensive correction order, which is a community-based sentence.

But Crown prosecutor Keegan Lee pushed for a term of full-time imprisonment, and Acting Justice McWilliam ultimately found this was the only appropriate option.

She said the offender could not rely on factors like his prior good character because it was this that had helped him gain access to the victim and commit the offence.

"The conduct is worthy of denunciation and I would not consider an [intensive correction order] appropriate," Acting Justice McWilliam said.

The man will become eligible for parole in March 2023.

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