A judge has described a BMX rider who sexually abused more than 20 young girls as a "monster of sexual depravity", whose crimes demonstrated the dangers of social media.

Fabian Meharry, 28, befriended 22 girls on social media websites before asking them to send him sexual photos of themselves.

The judge called Fabian Meharry a "monster of depravity" and jailed him for 12 years.

He blackmailed the girls, aged between 11 to 17 years, into performing degrading and painful sexual acts on themselves online, secretly recording videos of them which he used to continue blackmailing them.

County Court Judge Liz Gaynor sentenced Meharry on Wednesday to 12 years' prison with a non-parole period of 10 years for more than 60 child sexual abuse crimes including producing child pornography, sexual penetration of a child under 16 and grooming children using a carriage service.