SYDNEY, March 31 (Reuters) - The Australian internet regulator said it was investigating five of the biggest social media platforms for suspected breaches of its new under-16 ban, its strongest signal yet that companies may face enforcement action under a world-first regime.

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 non-compliance and the watchdog was gathering evidence for possible penalties.

"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," Inman Grant added.

Under the Australian law, platforms face a fine of up to A$49.5 million ($34 million) for noncompliance, and the regulator added on Tuesday they also faced reputational damage if found in breach of the law.

eSafety said it found major gaps in the way platforms were complying with the ban, including prompting children who had previously declared ages under 16 to do fresh age checks, allowing repeated attempts at age-assurance tests until a child obtained a result over 16, poor pathways for people to report underage accounts, and insufficient safeguards to prevent new under-16 sign-ups.

The regulator said each platform had been notified of specific concerns and expectations for improvement.

TikTok declined to comment, while spokespeople for Meta, Snap and Google were not immediately available for comment.

($1 = 1.4599 Australian dollars)