Platforms such as Facebook, Snapchat, TikTok and YouTube were already included in the ban -- a world-first.

But the head of Australia's eSafety Commissioner, Julie Inman Grant, has written to 16 more companies to "self-assess" whether they fall under the ban's remit.

The list also included Pinterest as well as Lego Play, streaming company Kick and gaming platform Steam.

Companies will need to make a case if they believe their platform should be exempt from the ban, the national broadcaster ABC said.

Inman Grant told the network that while some cases were "pretty clear", the regulator would "give them the due diligence process".

"We need to hear them all out," she said.

The regulator said it will initially focus on platforms with the greatest number of users, where there are higher risks of harm.

A Roblox spokesperson insisted the platform was not a social media company and therefore did not qualify for the ban.

"We prohibit users from uploading real-world photos or video, or re-sharing news, and we do not offer social media feeds within experiences in Australia," the spokesperson told AFP.

"We completed the self-assessment process and communicated to eSafety that our position remains that we are an exempted online gaming platform," they added.

'Vague' regulations

Australia has been a leader in global efforts to prevent internet harm, but current legislation offers almost no details on how the ban will be enforced -- prompting concern among experts that it will simply be a symbolic piece of unenforceable legislation.

The eSafety Commission will be able to fine social media companies up to Aus$49.5 million (US$32.6 million) for failing to comply with the rules.

Social media companies have described the laws as "vague", "problematic" and "rushed".

Canberra has struggled to pin down how precisely platforms will verify the ages of their users.

This month, Communications Minister Anika Wells conceded that firms would need to self-regulate.

An independent study ordered by the Australian government found this month that age checking can be done "privately, efficiently and effectively", though it admitted no single solution would fit all contexts.

The regulator has also introduced a number of rules taking effect in Australia in the coming months to protect children from "lawful but awful" content, including online pornography and AI chatbots capable of sexually explicit conversations.

This month, Roblox agreed to curb the risk of adults grooming children on its platform in Australia.