A Sydney man who shared "revolting" online fantasies about children and expressed a sexual interest in a seven-year-old girl has avoided jail.

Last October, Julian Robert James Sear, 44, began talking on an adult website to an undercover female police officer, who was posing as a member of the public.

Sear told the woman in a series of conversations, which included Skype calls, that he had a sexual interest in female children.

The woman said she had an 11-year-old daughter and Sear detailed sex acts he said he would like to perform on the girl.

He also said he had a sexual interest in the seven-year-old girl who lived in the area and sent a non-pornographic image of her to the woman.