Daniel Watson, 33, from Broadmeadows, pleaded guilty in the County Court on Tuesday to a number of child pornography charges where he would pose as either a teenage girl or boy to convince young girls to send him sexually explicit photographs and videos.

If a young girl refused to continue to do what he told her, he would threaten to post the images on Facebook and other social media outlets or send them to the victim's parents and school.

Prosecutor Andrew Sprague said Watson targeted more than 70 girls between 2011 to 2013 and received 856 child pornography images and 47 videos.

Watson would contact the victims, who came from across Australia and overseas, online and ask if they wanted to have some fun.

He would then send them a sexually explicit photo of a teenage girl and ask them to take a similar photo of themselves. When they did, Watson would use the photos as leverage to get more images and videos from them.

Acting Chief Judge Michael McInerney said it was amazing how in today's society all these young children had iPhones and iPads which they used to go online and their parents had no control over what they were doing.

Judge McInerney said he was surprised young girls could be persuaded to send naked and sexually explicit photos of themselves to a complete stranger.

A 15-year-old girl and her five-year-old sister living in America were two of Watson's victims.

Watson, posing as a teenage boy, convinced the teenage girl to send him sexually explicit photos and then demanded she send him photos of her with her sister.

Watson told the teenager that if she blocked him, he would report her to the FBI.

Mr Sprague said Watson, posing as a teenage girl, also preyed on a 14-year-old girl and her sister, 10, living in Victoria.

The prosecutor said the 10-year-old felt she had no choice but to do what Watson wanted and make the "disgusting pictures" because otherwise her older sister's future would be ruined, given he was threatening to post images of her on the internet.

Watson would tell his victims, "If you don't do it, I'm going to finish you now."

Defence lawyer  Amelia Beech said Watson's life could be characterised as one of loneliness, isolation and victimisation.

Ms Beech said that before she died from cancer when Watson was 18, his   mother was forced to move him to five different primary schools and five different secondary schools because he was constantly being bullied.

She said Watson retreated into the online world because it was a safer place for him where he didn't have to face people.

Watson's only real relationship with a woman ended in his early 20s when she had an affair with another woman.

He then met another woman on a gaming website and they exchanged naked photos online before she posted the images on the site and made disparaging remarks about him.

Ms Beech said he was devastated at this betrayal because the internet had been the only place where he had not been criticised for his physical appearance.

She said Watson's view of the world became warped so he increasingly turned to alcohol and the online world and this was the context in which his offending occurred.

Watson was remanded to be sentenced at a later date.