Tuesday's announcement marks the government's first public assessment of compliance with the law that is being studied by policymakers globally. Weak adherence by the biggest platforms could undermine the momentum of governments considering similar restrictions.
eSafety Commissioner Julie Inman Grant said Meta's Facebook and Instagram, Snapchat, TikTok and Google's YouTube had been flagged for potential noncompliance and the watchdog was gathering evidence for possible penalties, with a decision by mid-year.
"While social media platforms have taken some initial action, I am concerned through our compliance monitoring that some may not be doing enough to comply with Australian law," she said in a statement.
"We are now moving into an enforcement stance."
Under the Australian law, platforms face a fine of up to $A49.5 million ($NZ59.2 million) for noncompliance, and the regulator added on Tuesday they also faced reputational damage if found in breach of the law.
The global push for greater regulation of such platforms has gained momentum after courts in the United States found social media companies negligent for designing addictive algorithms that hurt children.
Australia's eSafety initially hailed the ban's success, saying in the initial weeks the platforms took down 4.7 million suspected underage accounts, and on Tuesday it said platforms had stopped another 300,000 underage accounts being activated.
But the regulator reported compliance gaps, including platforms prompting children who had previously declared ages under 16 to do fresh age checks, allowing repeated attempts at age-assurance tests until a child got a result over 16 and poor pathways for people to report underage accounts.
Some platforms did not use age-inference, which estimates age based on someone's online activity, and some only used age-assurance measures like photo-based checks after a user tried to change their age, rather than at sign‑up.
That made it "likely many Australian children aged under 16 have been able to create accounts on age‑restricted social media platforms by simply declaring they are 16 or older", the regulator said.
Communications Minister Anika Wells said the alleged breaches were unacceptable.
"The kind of tactics we're seeing deployed by social media platforms to undermine Australia's world-leading law are right out of the big tech playbook.
"If eSafety finds these companies have systemically failed to uphold their legal obligations, I expect the commissioner to throw the book at them."
Prime Minister Anthony Albanese has long conceded not every young person would be kicked off social media, but enough would be for the restrictions to be effective.
A number of other countries are now considering following suit and barring children from social media, including Austria, France, Denmark, Spain, Greece and Malaysia.
Indonesia also began blocking under 16s from having accounts on YouTube, TikTok, Facebook, Instagram, X, Bigo Live and Roblox on Saturday.
- Reuters and AAP
