A man accused of knocking out an eight-year-old girl with a chloroform-filled mask wanted to "act out his own sleep-assault fantasy", a prosecutor said on Thursday.

But Thomas William Johnston denied stupefying the girl, saying he would never hurt her.

The 51-year-old was on trial in the ACT Supreme Court, having pleaded not guilty to administering a stupefying or overpowering drug, poison or injurious substance likely to endanger life or cause grievous bodily harm.

Johnston also pleaded not guilty to an alternative count of assault occasioning actual bodily harm, and two counts of using a child to produce pornography.

He admitted deliberately possessing child pornography, and on Thursday told the court he was addicted to adult pornography.

"It was a part of my life that I was extremely disgusted with, not just the child side of it but the adult side of it," Johnston said.

The Crown alleged the chloroform incident occurred when the eight-year-old girl, a family friend, was spending an afternoon at Johnston's house in November 2009.

The girl would later tell her mother and police that Johnston put a mask on her, causing her to smell a strong odour and making her eyes water.

She said she soon fell asleep and woke up in a different part of the house, and was still feeling nauseous the next day.

Johnston told the court she was coughing so he put garlic on the inside of a mask and got her to wear it before the pair fell asleep watching television, and he later moved her from the couch to the bed.

But when police raided his house several days later they discovered a bottle of chloroform in his bedroom, child pornography on electronic devices and child porn websites in the favourites list of a web browser.

The accused man said he thought the liquid was acetone, a flammable liquid, and he kept it on the floor of his room because he didn't want it to fall and smash.

Johnston also said he was unaware chloroform was used as an anaesthetic in the past, and he thought it had something to do with chlorine.

The Crown argued that a series of images taken on his mobile phone depicted either the eight-year-old girl or her younger sister.

Johnston told the jury he had been trying to overcome his problem with pornography, and had intended to throw out a disk full of explicit images, including child porn.

But prosecutor Kylie Weston-Scheuber pointed out the disk was still sitting prominently on his computer desk two months after he decided to get rid of it.

He said it was "sheer neglect … it was one of those things put in the 'when I get around to it' basket".

The accused man also said the fact an image of a young girl naked had been set as the screensaver on his hard-drive was an accident.

Police also found several pornographic images depicting "sleep assaults" on females.

But Johnston denied sleep assaults were a particular fetish of his, and denied obtaining the chloroform to act out such a fetish on the alleged victim.

In her closing address to the jury, the prosecutor submitted the case against Johnston was "frankly, overwhelming".

Defence barrister Ken Archer was due to address the jury on Friday, and the jury was expected to begin deliberations after that.