The offender, who is not identified to protect the victim's identity, pleaded guilty to an indecent act against the child under 10 years old.

The man, in his 40s and currently living in Queanbeyan, faced an ACT Supreme Court sentencing on Wednesday when it heard he told police he committed the 20-second act to get a reaction from her.

The victim's pants and underwear came off during the incident and she experienced physical pain before she was taken to hospital for examination.

The impact statement by the victim's mother was read to the court.

"Since this has happened to my daughter and family, I have felt broken, empty and devastated," the statement reads.

"There have been so many heart-breaking moments since finding out what [the offender] has done.

"He used that trust to hurt her for his own pleasure disregarding the impact this will have on her life now as well as later.

"The impact that this will have on her mental health and relationships with men for the rest of her life is concerning for me."

The victim's mother wrote that her daughter was already suffering from separation anxiety and no longer felt safe sleeping.

She said she, too, had increased anxiety, often feeling overwhelmed and in an increased state of hypervigilance.

"Many tears have been cried ... many sleepless nights wondering why this happened," she said.

"[The offender] took away so many things from our family, but there is one thing he can't take and that is seeing the strength of my daughter."

The court heard that the victim disclosed other incidents to police to which the offender denied.

Those other alleged incidents were uncharged because of an "academic agreement" between the prosecution and the defence.

Defence lawyer Benjamin Rutzou said he and the prosecution agreed to leave those incidents as uncharged after Mr Rutzou agreed to not use the single indecent act as an isolated offence and thereby a mitigating factor.

Mr Rutzou said his client maintained it was not for sexual gratification despite it having a sexual connotation.

He said the latter did not mean the former and that there was no other evidence about how he touched her.

"He said he didn't realise what he did was wrong," Mr Rutzou said.

"But he also claims he knew he crossed the line and it was inappropriate the way in which he touched the victim."

He cited the offender's lack of criminal history and apology to the victim as mitigating factors and argued for a community-based sentence rather than actual custody.

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Crown prosecutor Keegan Lee argued against a community-based sentence, saying it may not reflect the gravity of the offending.

Mr Lee said no other explanation existed for the offence other than one of sexual gratification.

"Twenty seconds is quite a lengthy time ... there's no rational explanation of what he's done. Why did the child's pants come off? Why did her underpants come off?" he asked.

"The licking of the finger - that was a highly sexualised conduct. That's not in anyway explained by the reference to roughhousing."

Mr Lee said it was aggravated by the breach of trust as he was in a position of care and that there had been "clearly a profound effect on the victim and her mother".

He urged Acting Justice Verity McWilliam to reject what the offender told police about why he offended.

"It does not make sense, it does not add up," Mr Lee said.

In her preliminary sentencing remarks, Acting Justice McWilliam said it was "a mother's worst nightmare" and that there must have been some force as the child said it hurt.

She will deliver her sentence on September 10.

Correction: An earlier edition of this article referred to Acting Justice Verity McWilliam as Associate Justice. This has now been updated.

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