Facebook, Instagram, Snapchat, TikTok and YouTube may have allowed children under 16 to access their apps after the age restrictions took effect in December, Communications Minister Anika Wells said in a statement.
Some children have been allowed to create new accounts straight after their previous one was deactivated, or repeatedly attempt age verification until they pass, the online watchdog has found.
Other platforms did not provide appropriate pathways for underage users to be reported, the eSafety Commission said in its first compliance report since the ban kicked in.
Ms 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," she said.
"If eSafety finds these companies have systemically failed to uphold their legal obligations, I expect the commissioner to throw the book at them."
Systemic breaches of the age restriction laws can be punished with fines of up to $49.5 million.
When the ban took effect, some children said their accounts had not been deactivated.
Others reported being able to bypass age verification with relative ease.
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.
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