Matthew O'Hara adopted the persona of a 12-year-old girl called Maddie to befriend his victims on Twitter, then began sexual conversations with them in October last year.

The four girls, aged between eight and 13, live in the United States, the Philippines and Queensland, the WA District Court heard on Tuesday.

In the case of the eight-year-old girl, she sent O'Hara nude photographs, while he shared a link to a Twitter page that showed lesbian sex.

O'Hara, who has never been in a sexual relationship, was caught in November last year following an Australian Federal Police investigation into child exploitation material on Twitter, the court heard.

During a search of O'Hara's home, police found hundreds of child pornography videos and images on devices, and he admitted contacting girls online.

Chief Judge Kevin Sleight said O'Hara had collected about 12 pictures of a girl for Maddie's profile.

"You essentially stole the identity of a young girl from another Twitter account," he said.

"You present as a lonely, vulnerable person who reverted unfortunately to accessing child pornography and engaging in communications with children on Twitter ... to meet your emotional and sexual needs.

"You engaged in conduct which was likely to cause confusion as to their sexuality, and certainly distort their moral and appropriate sexual behaviour compass."

Judge Sleight said a court-ordered psychological report indicated O'Hara had little appreciation of the harm suffered by the children and he was at risk of reoffending.

But he noted O'Hara was receiving psychological treatment.

O'Hara, who pleaded guilty to a string of state and federal charges but has no prior criminal record, will be eligible for parole.

Australian Associated Press