When the girl blocked him, Amarasinghe messaged her from new accounts, threatening to pass on the abuse content to her friends and family if she did not comply with his demands for even more material.
Her mother begged the 24-year-old, who often posed online as "John", to stop, saying she feared her daughter might take her own life because of the trauma he had inflicted upon her.
But he told her that he hoped she did just this.
Amarasinghe on Wednesday faced the Victorian County Court, where he was sentenced to a total 13 years and six months in jail.
The Sri Lankan national pleaded guilty to 25 child abuse material offences relating to six girls aged 11 to 17, including four in the United States, one in the United Kingdom and one in Australia, from November 2018 to June 2020.
"Get ready to be ruined," Amarasinghe told one girl.
"I'm ruining your life and you don't give a s*** - you're a f***ing whore."
Judge Douglas Trapnell said Amarasinghe had been on a cruel and perverted mission to completely destroy people's lives.
The 24-year-old's "insidious" and "chilling" actions left the girls and their families devastated by extreme trauma, he added.
"Your depravity knew no bounds," Judge Trapnell told the court.
"It was bad enough forcing these young, innocent girls to sexually compromise themselves. But to threaten them in the way you did and carry through with it was nothing short of brutal.
"You presented one face to the world as a reasonable person but hid behind a cloak of anonymity while acting as a sexual predator online. By sexualising these girls you dehumanised them and in the process lost your own humanity."
Amarasinghe used social media apps such as Kik, Omegle, Snapchat and Instagram to connect with the girls online.
He used several fake aliases, would often say he was a teenager, and usually followed through with his threats to share the child abuse material with the girls' friends or followers.
"Your offending was protracted, planned, organised and prolific," Judge Trapnell said.
"You played an active, not simply a passive role in the child (abuse) market."
Amarasinghe, once a promising a cricketer, had watched porn online from the age of 11, struggled to develop intimate relationships and had a "fragile self-esteem", the court was told.
He was arrested in September 2020.
"Sextortion, also known as image based abuse, is a form of blackmail where someone threatens to share intimate images of you online unless you give in to their demands," Australian Federal Police detective acting superintendent Aaron Hardcastle said in a statement following the sentencing.
"In this case, not only are the girls deceived into self-producing child exploitation material that could be circulated to other predators, it has been weaponised against them in a campaign to punish and shame them for stopping further exploitation."
Amarasinghe must serve at least eight years and six months behind bars before he is eligible for parole.
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