* Regulator says Meta, YouTube, TikTok under

investigation

* Age inference, age assurance underused by platforms,

regulator says

* Government accuses platforms of using "big-tech

playbook" to skirt ban

(Adds report detail in paragraphs 8 and 10, communications

minister in paragraph 13)

By Byron Kaye

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 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," Inman Grant

added.

TikTok declined to comment, while spokespeople for Meta, Snap

and Google were not immediately available for comment.

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.

The global push for greater regulation of such platforms has

gained momentum after U.S. courts 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 accused the platforms of

using tactics "right out of the big-tech playbook ... to

undermine Australia's world-leading law".

($1 = 1.4599 Australian dollars)