A man arrested over child pornography had more than 19,000 illegal images and videos on his computer, a court has heard.

Stephen John Foster, 63, was arrested at his Ethelton home last January after South Australian police received advice from United States authorities.

Prosecutors told the Adelaide District Court an undercover special agent in New Mexico downloaded the images Foster had sent via a file-sharing program.

The court heard when police seized Foster's computer they found the thousands of images and videos of child pornography.

Prosecutor Nicholas Wong detailed the extent of the material.

"The defendant was in possession of 18,100 images and 1,060 videos depicting child pornography. This was a substantial amount of child pornography material," he said.

Mr Wong said the special agent found 637 images Foster had uploaded, which could then have been accessed by others.

"They were there for others to pick up as they chose," he said.

"This is serious offending which calls for the imposition of a custodial sentence.

"General deterrence as well as specific deterrence must be a paramount consideration for offending of this type."

Prosecutors said most of the images were classified as category-one, at the lower end of the scale, but some ranged through to category five which was considered the most serious type of child exploitation material.

Defence lawyer David Stokes urged mercy.

"Like many people in this position he has over-downloaded and not got around to deleting what he is not interested in," he told the hearing.

"I accept that general deterrence looms large. Because of the number of images concerned, imprisonment is more likely than not, however there will be cases which justify mercy. He is, we say, such a person.

"He's not interested in small children, he was caught out by the scale of the offending and was caught out by the number of images. His real interest is in older teenagers. This man is not a risk to children, he is not a risk to the community.

"The process of going through the court proceedings has been adverse enough. I urge Your Honour either to suspend the term of imprisonment or impose a minimal non-parole period."

Foster will be sentenced next week.