New details of Stephen James Porter's crimes can now be revealed after an agreed statement of facts was tendered to the ACT Supreme Court.

The 51-year-old Macgregor man is before that court for sentence, having pleaded guilty to four child sex-related charges.

His fate will not be known until after Justice Chrissa Loukas-Karlsson rules on a factual dispute about the most serious offence, which is that Porter had a sexual relationship with a boy between 2015 and 2018.

The offender approached this child's parents while coaching at the Ainslie Football Club and arranged to mentor the boy privately, later using his time alone with the victim to sexually abuse him.

He claims there was inappropriate sexual contact on no more than 15 occasions, while prosecutors allege the correct number is somewhere between 35 and 40.

Few details of Porter's other three crimes have been aired publicly, but facts agreed by prosecutor Andrew Chatterton and the offender's solicitor, Adrian McKenna, shed light on them.

The agreed facts show Porter worked in information technology for the ACT government, but he also coached Australian rules football in volunteer roles.

He began coaching his first victim, a boy he used to produce child exploitation material, at the Belconnen Magpies in 2009.

This child would regularly attend Porter's home, where the offender hid cameras to film the boy doing things such as getting changed.

A summary of all the videos and images Porter amassed relating to this boy, over a period in excess of two years, stretches across more than 16 pages.

The files were found by police who searched the 51-year-old's home in June last year, following a complaint made by the victim of the unlawful sexual relationship.

Investigators' searches of his devices also uncovered messages that revealed Porter was grooming a third boy right up until police knocked on his door.

Porter's messages showed he had recently contacted the mother of a young Ainslie Football Club player, offering private coaching sessions and referring to how he had done these for many others over the years.

"I'm a coach 24/7," he wrote in a March 2020 message. "It's in the blood I think now."

The woman accepted Porter's offer and he began training her son at a public oval during the ACT's first lot of coronavirus restrictions, developing a more personal relationship with the boy as time went on.

This boy attended Porter's home on one occasion with other members of a team, but other invitations for him to go to his place were declined by the child's parents.

Throughout the initial stages of Porter training this boy, the offender sent a friend of his in the UK messages about the child.

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He referred to how his sessions with the boy reminded him of the early days of training the first two victims, describing how he was "falling so hard" and saying he was "completely head over heels".

"Mate I can't believe that some Chinese dude eating a bat has eventually led to me spending so much time with a special b," one message to the Brit said.

The final offence Porter has admitted is one of possessing child exploitation material, which was stored across seven of his devices.

This charge relates to more than 1500 videos and pictures of mostly unidentified boys, including some in which the subjects are engaged in sexual activity with men.

A child getting changed in one of the videos has been identified as a boy whose family Porter and his friend from the UK visited in Queensland on two occasions.

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