Australia's internet regulator said a teen social media ban would be the first domino to fall in a global push to rein in Big Tech, as Meta's Instagram, Facebook and Threads began locking out hundreds of thousands of accounts ahead of a deadline next week.

eSafety Commissioner Julie Inman Grant said she had initially expressed concern about the "blunt-force" approach of blocking under-16s from social media but she had come to embrace it after incremental regulatory changes were not effective enough.

"We've reached a tipping point," Inman Grant said on Thursday at the Sydney Dialogue, a cyber summit.

"Our data is the currency that fuels these companies, and there are these powerful, harmful, deceptive design features that even adults are powerless to fight against. What chance do our children have?"

Governments around the world were watching as the Australian law takes effect on December 10, and "I've always referred to this as the first domino, which is why they pushed back", she added, referring to the platforms.

After more than a year campaigning against the ban, which carries a fine of up to A$49.5 million ($33 million), platforms owned by Meta, TikTok, Snap's Snapchat and Alphabet's YouTube have said they will comply.

Some 96% of Australian teenagers under 16 - more than a million of the country's 27 million population - have social media accounts, according to eSafety.

Although the law takes effect on December 10, Meta's Instagram, Facebook and Threads began deactivating accounts from Thursday, according to screenshots seen by Reuters.

Most other affected platforms have started contacting underage users advising them to download their photos and contacts and offering the choice of deleting their accounts or freezing them until they turn 16.

"It's a great thing and I'm glad that the pressure is taken off the parents because there's so many mental health implications," said Jennifer Jennison, a Sydney mother.

"Give my kids a break after school and they can rest and hang out with the family."

At the conference, Inman Grant said lobbying by the platforms had apparently involved taking their case to the U.S. government, which has asked her to testify at its congressional House Judiciary about what it called an attempt to exert extra-territorial power over American free speech.

Inman Grant didn't say if she would agree to the request but noted that "by virtue of writing to me and asking me to appear before the committee, that's also using extra-territorial reach".