Confessed child abuser Stephen James Porter, of Macgregor, faced the ACT Supreme Court on Thursday for the start of a disputed facts hearing.

The Australian rules coach has pleaded guilty to four child sex charges involving the abuse or grooming of three boys and the possession of child exploitation material.

The 51-year-old's offending took place over more than a decade between 2009 and his arrest in mid-2020.

While Porter has pleaded guilty, the details of his unlawful sexual relationship with one of the boys are the subject of disagreement.

Crown prosecutor Andrew Chatterton said on Thursday he alleged the 51-year-old had engaged in sexual activity with this particular victim between 35 and 40 times over the course of about three years.

But the position of Porter's barrister, Jack Pappas, is that the abuse occurred "no more than 12 to 15 times" between 2015 and 2018.

Mr Chatterton said there was also controversy surrounding how many times a particular type of sexual activity happened.

In a courtroom closed to the public, with the exception of journalists and a witness intermediary, he began to play a video of this victim's interview with police.

The boy, who is now a young adult, told a constable he was playing Australian rules at the Ainslie Football Club when Porter, a coach there, approached him and his parents.

Porter did not coach this boy's team and they had not previously met.

"He said I had a bit of talent and he wanted to develop that," the victim said.

The boy went on to describe how, about a year into a private coaching arrangement with Porter, things began to get inappropriate.

Porter started talking about how "he used to do things to make other people he developed feel good".

The victim said Porter named other boys, including one the victim looked up to, before saying he hoped he would be able to do the same things with him one day.

Eventually, one day when the pair were playing Xbox games at the offender's home in Macgregor, Porter put his arm around the boy and led him to a bedroom.

"He said to me, 'You're not going to regret this'," the victim told police.

The boy recounted the graphic details of what happened on that first occasion, then began to speak about what had gone on as time passed.

He told police Porter had grown closer and closer to his family during the abuse, to the point the offender would come over for dinner and sometimes stay the night.

Once the boy's parents went to bed, Porter would molest him in his own Canberra home.

The victim, who was in his early teenage years at the relevant time, spoke of physically shaking at stages during the abuse he suffered.

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"I'm nervous about whether people will believe me or not," he told the the constable.

The disputed facts hearing is set to continue on Friday, when the remainder of the victim's roughly three-hour interview with police will be played. He will then give evidence.

It will ultimately be up to Justice Chrissa Loukas-Karlsson to rule on the disputed matters before she sentences Porter, who is currently on bail.

Justice Loukas-Karlsson said on Thursday the offence relating to this victim - maintaining a sexual relationship with a child - was serious regardless of the number of incidents.

"I'm trying to understand, for my own part, whether or not there would be any significant difference on sentence in circumstances where this is already a very serious charge ... even on the lower number," the judge said.

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