The 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.
Australia in December banned users under 16 from the world's largest social media platforms, citing the need to protect them from "predatory algorithms" and online bullying.
Inman Grant said there were "significant concerns" that Facebook, Instagram, Snapchat, TikTok and YouTube may have breached the ban.
"As a result, we are now moving into an enforcement stance," she added.
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.
(With inputs from agencies)
