Scott Ian Whittaker, 39, fronted sentencing in the ACT Supreme Court on Thursday after pleading guilty to 25 charges that include having a sexual relationship with a child, numerous counts of having sex with someone under 16, sending indecent material, production of child exploitation material and indecent acts.

All of those offences were against the one victim.

Whittaker, who was a registered child sex offender for almost all of the period between 2002 and 2018, also pleaded guilty to possession of child abuse and exploitation material, as well as a firearms charge.

An agreed statement of facts reads that the relationship began in late 2017 when he started sending the victim, under 16, messages on Snapchat, including about how he loved her.

"You're just so perfect, and I [sic] words do not do justice to just how much I love and adore you," he said.

Whittaker, of Bonner, also took seven photos of him and the victim hugging and kissing each other before sending them to her.

Other videos he sent her on Snapchat included a man masturbating.

On Valentine's Day, he sent her a lengthy message again about his love for her.

"You're the person I've done all my firsts with and I couldn't be happier that it was with you...Eternal happy Valentine's Day baby girl," he said.

Between March and December of 2018, he recorded seven videos of him having sex with her and performing indecent acts against her.

In relation to the possession of child abuse and exploitation material, he was caught after he stored the material on a New Zealand-based encrypted cloud-storage website.

In late November 2019, the New Zealand Department of Internal Affairs notified the Australian Centre to Counter Child Exploitation about Whittaker's email being identified as accessing and saving child abuse material.

The account was examined and found to have stored files that contained such material.

Following police investigation, they attended Whittaker's workplace in Fyshwick and searched it before accompanying him to his house where they searched his two phones and computer.

During examination of a phone, police found 551 video files within a "Photo Vault" app.

Of those videos, 383 were deemed child abuse material, involving at least 100 children between 18 months and 15 years old.

Police searched his computer and found numerous photos and videos of the victim Whittaker had sex with.

They also found two rifles, magazines and bullets stored in a transport case in his room.

The case is not an approved type at residential premises and Whittaker had his firearms licence suspended and weapons seized.

During an interview with police in March this year, Whittaker did not comment about the material found on his devices and his relationship with the victim.

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In her sentencing remarks, Chief Justice Helen Murrell said it was clear that the only appropriate sentence was a significant jail term as "the evil associated with engaging in a sexual relationship extends beyond that associated with individual sexual violations".

"The very significant age difference bespeaks a heavy power imbalance and aggravates the offender's culpability," she said.

"I infer that the complainant has suffered a very significant psychological injury."

"Although the offender's prospects for rehabilitation are unclear, the total sentence is lengthy," she said.

Chief Justice Murrell dismissed defence lawyer Justin Raine's argument for leniency because Whittaker gave police passwords to his devices and admitted to them about some of the offences.

"The admissions were not extensive ... the offender was legally required to provide passwords," she said.

A pre-sentence report assessed Whittaker as being above average risk of sexual reoffending and said that he had the relationship because he was feeling isolated and lonely.

He told the report author that each of his past relationships had been an "absolute disaster" because he found it difficult to communicate his emotions.

The jail term of 9.5 years has a non-parole period of four years and nine months.

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