Australia's internet watchdog is investigating five major social media platforms for allegedly not complying with a world-first social media ban for under 16s, the government in Canberra said on Tuesday.

Under the new law, which took effect in December, under 16s are no longer allowed to have their own accounts on 10 major social media platforms.

Communications Minister Anika Wells said that the eSafety Commissioner was "actively investigating potential non-compliance in relation to five platforms: Instagram, Facebook, Snapchat, TikTok and YouTube."

Wells said that a "concerning" number of children under 16 was still accessing social media despite the ban.

The minister said that the first report on the ban by the eSafety Commissioner exposed "unacceptable systems" being used by tech giants, including allowing users to repeatedly attempt to pass age verification checks.

"The kinds 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," said Wells in a statement.

"This new report from eSafety Commissioner shows that social media giants seem to be trying to get away with doing the bare minimum – I have serious concerns about their compliance with the law," the minister continued.

"If eSafety finds these platforms have systemically failed to uphold their legal obligations, I expect the Commissioner to throw the book at them."

Wells said that, as of early March, 5 million under-16 social media accounts have been removed, deactivated or restricted.

Tech giants could be fined up to $49.5 million Australian ($33.9 million US) if they do not comply.