A South Australian man charged with child pornography offences after a tip-off from the FBI has been told by a judge to prepare for time in jail.

Mount Gambier man Gary Michael Johnstone, 54, was arrested after the FBI contacted the Australian Federal Police (AFP) about a child pornography ring.

Prosecutors say Johnstone was frequently in contact with people whom he believed were abusing children at his request.

Police allegedly found 658 illicit images and 33 videos on Johnstone's computer equipment after they searched his house.

He has pleaded guilty to accessing and sharing child pornography on the internet and making indecent communications with a child.

South Australia's District Court today heard Johnstone had made sexually suggestive comments to an 11-year-old girl through an internet chat room, the electronic logs of which were also stored on his computer.

Prosecutor Tim Griffin told the court Johnstone regularly emailed people who sent him pictures of children under their care.

"Mr Johnstone at least believed that the adult on the other end of the email exchange had a child available," he said.

Judge Paul Cuthbertson said that throughout the email exchanges Johnstone actively encouraged the sexual abuse of a child.

"A disturbing aspect is that [Johnstone] is actually contemplating that there is some poor little kid somewhere that's being subject to, and over a period of time he's encouraging it, sexual abuse," he said.

"He says in one of his emails, 'I'd like to see some pics of the girl' and in one of them he says, 'and have you found someone to have sex with her yet?'."

'Grave offending'

The court heard Johnstone immediately admitted his conduct when the AFP arrived at his house with a search warrant.

Johnstone's lawyer John Dillon said his client developed depression after his mother's death and started viewing the material online.

He said his client was remorseful and was now undergoing rehabilitation.

Mr Dillon told the court Johnstone should receive a suspended sentence because of his health problems including the early stages of multiple sclerosis and a painful nerve condition in his face.

"He poses a negligible risk of reoffending and personal deterrence shouldn't weigh that heavily on this court's mind," he said.

"In terms of general deterrence, given the particular issues of this man, if the court simply sentences him to an immediate term of imprisonment it's doing more harm than good."

But Mr Griffin argued that an immediate jail sentence would be the only appropriate penalty given the extremely serious nature of the offending.

Mr Griffin said it was also concerning that Johnstone had propositioned a girl in an internet chat room.

"It is particularly grave offending. Parents can only be so diligent in trying to ensure their kids remain safe in an online environment, but with massive amounts of email access and smartphones and computers, they can't constantly monitor where their children are online and whether they are safe," he said.

"The courts need to send a strong message of deterrence to like-minded members of the community."

Judge Cuthbertson told Johnstone to prepare for a custodial sentence.

He was released on continuing bail ahead of his sentencing in April.