Christopher Cooksley, 72, told the ACT Supreme Court he fostered over 300 boys during the 70s and 80s

Christopher Cooksley, 72, took to the witness stand in the ACT Supreme Court on Thursday when he faced sentencing.

The NSW man was charged with one count each of maintaining a sexual relationship with a child and indecent assault on a male.

Cooksley pleaded guilty to both charges before the ACT Magistrates court on July 14.

An agreed statement of facts showed the 72-year-old committed the historical offences while residing in Canberra in the late 1970s and early 1980s.

The victim was 11 when the two met while Cooksley was working at a Holt service station in 1978.

Through spending years developing a close relationship, Cooksley eventually performed an obscene act on the boy and urged him to stay quiet in 1980 when the boy was 13.

"Sexual activity became a regular occurrence between the offender and the victim, happening two to three times a week at the offender's house until partway through 1981, over a period of approximately nine months," the agreed facts state.

On Thursday, prosecutor Murray Thomas read out the victim impact statement on behalf of the now adult man.

The victim described "having my innocence, trust and realm of reality crushed" by Cooksley's offending and the pillar of life the victim had built was "shattered".

The offending occurred for nine months and the victim said it still affected him almost every day.

"In the last 44 years there has been barely a day that doesn't pass without effects due to my childhood trauma," the impact statement read.

"Time does not remove the heartache, misery and anguish."

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The victim described turning to drugs and alcohol to cope with the trauma during a period when "you just didn't talk about those things" and never had the tools or mechanisms to manage.

A "pivotal life-saving moment" for the victim was the conception of his daughter and he felt he could not be a parent without fronting what happened when he was a child.

The victim came forward to police, saying he left his "dignity and pride on the door step".

When police charged Cooksley, he confessed to the relationship he had with the victim and served six months in 2009 at a NSW prison for assaulting the victim after driving him to the South Coast.

The offender also confessed to police that he had a relationship with another 15 year-old boy during the 80s.

During Thursday's sentencing Cooksley was cross-examined about the details of the case, revealing he fostered over 300 boys, often at the younger end of ages 11 to 18 during the 80s.

Cooksley said this was because "the damage is already done" when fostering older boys while "you can fix things" for younger boys.

While the ACT denied his request, Cooksley was able to foster a large number of the boys when he resided in NSW.

He said he never sexually assaulted any of the boys he fostered but admitted he had a physical attraction to some of them when they lived in his home.

When questioned by defence lawyer Greg Hoare, the offender admitted he "didn't understand" why his offending was wrong at the time but now has a better understanding.

During cross-examination Mr Thomas suggested the offender was only thinking of himself and failed to consider what was best for the boy.

However Cooksley said he "didn't force [the victim] to do anything" and he "[cared] about him a lot".

At the end of cross-examination, the prosecutor applied to revoke bail which Justice Chrissa Loukas-Karlsson granted as "adopting a realistic perspective, jail is inevitable".

Cooksley was taken to custody and will face the Supreme Court for closing submissions on November 11.

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