The university student had previously been charged with offences including sexual activity with a child and intimidation to commit an act of sexual activity with a child, between January 2018 and July this year.
McCartney was taken back into custody, and the case adjourned until January 15.
Police have previously confirmed a 12-year-old girl from Hawke's Bay was among the huge number of children across the world targeted.
Detective Sergeant Heath Jones warned parents to be alert to their children's use of the internet, particularly via mobile phones, which enabled predators to "target children from anywhere, at any time".
"To help your children stay safe you can also make sure you know who your children are making contact with online and which social networking sites they're using," he said.
"There now exists a wealth of information online to help parents to have a conversation with their children about online safety."
McCartney's case has been described as the biggest catfish child abuse investigation in UK court history. A prosecutor told the BBC they expected to identify thousands of new victims across the world.
The lawyer alleged the accused's modus operandi was to "blackmail young victims in to abuse" before trying to get their younger siblings or cousins involved.
Where to get help:
• If it's an emergency and you feel that you or someone else is at risk, call 111.
• If you've ever experienced sexual assault or abuse and need to talk to someone call the confidential crisis helpline Safe to Talk on: 0800 044 334 or text 4334.
• Alternatively contact your local police station
• If you have been abused, remember it's not your fault.