His victims were aged between 11 and 17 at the time.
But Meharry, who was 28 when jailed in March, is now facing a 22-year sentence, following a successful appeal by the Office of Public Prosecutions to Victoria's Court of Appeal.
He must now serve 18 years before he is eligible for parole.
The Court of Appeal found Meharry's initial sentence was manifestly inadequate for crimes that showed "a level of depravity rarely seen in the courts".
The court found the penalties imposed by County Court judge Liz Gaynor "simply did not reflect the objective gravity of the offending, or the profound impact on the victims".
The court's ruling comes a fortnight after it significantly increased the jail term of a man who got his 13-year-old stepdaughter pregnant.
That decision followed the High Court finding Victorian courts erred in giving too much weight to current sentencing practices and not enough consideration to the effects of crimes on victims and their families.
Meharry, who was was well-known in the Echuca BMX scene and had a significant online presence through a business he ran selling wax for skateboards, befriended the girls on social media websites, but had them perform degrading and painful sex acts on themselves while he secretly filmed and recorded them.
He later threatened to use the recordings or photographs he had the girls send him to post online or send to the girls' families, unless they continued fulfilling his requests.
He also told some of the girls that he would take his own life if they did not comply.
He had some girls perform acts of bestiality, and had one perform an incestuous act on a sleeping sibling.
Meharry had physical sexual relationships with three girls, one of which lasted 10 months. The girl in that relationship became pregnant and later had a miscarriage.
In victim impact statements, the girls reported feeling angry, saddened, disrespected, demeaned and having been stripped of their dignity.
Some had problems with anxiety, one had been diagnosed with post-traumatic stress disorder and another had self-harmed. Several had been unable to continue working or studying.
Meharry pleaded guilty to 62 charges including producing and possessing child pornography, sexual penetration of a child under 16 and grooming children using a carriage service.
Judge Gaynor, in sentencing, described his offending as callous, prolonged, "classically predatory" and remorseless, and labelled him "a monster of depravity".
The Court of Appeal said the judge was right to use strong language in describing Meharry's offending, but found there was "a striking disconnect" in the judge's remarks and sentence.
The punishment did not reflect his persistent and systemic abuse, and the vulnerability of the victims.
The appeals court said Meharry had shown little remorse or insight into his crimes.
A psychiatrist who assessed him found although he had a poor sense of self and a dysfunctional personality, there was little psychiatric explanation for what he did.
Meharry continued offending even after a police raid on his home in 2015, and failed to take heed of a 2008 warning from another judge, who convicted him for having a sexual relationship with a girl who was 13 when he was 18.
He had endured a difficult stint in jail, the Court of Appeal heard, after being targeted by other prisoners because of the notoriety of his crimes.