More than 80% of children in Australia aged 12 or under used social media or messaging services last year that are only meant to be used by over-13s.
The country's internet regulator eSafety found YouTube, TikTok and Snapchat were the most popular platforms used by young children.
It comes as Australia plans to implement a social media ban for under-16s that is expected by the end of this year.
The companies examined - Discord, Google (YouTube), Meta (Facebook and Instagram), Reddit, Snap, TikTok and Twitch - did not immediately respond to a request for comment.
