A jury has begun deliberating in the case of a man accused of knocking out an eight-year-old girl with a chloroform-filled mask.

Thomas William Johnston 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 told the court he was addicted to adult pornography.

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 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.

Defence barrister Ken Archer said the evidence before the court was not strong enough for a conviction.

Mr Archer said the number of images of children on Johnston's computer represented a small percentage of the overall - 40,000-plus items - pornography collection.

He said the overall pattern of usage had been distorted.

“It gives an impression of a pattern … that doesn't fit reality,” Mr Archer said.