They were instead assessed on whether their "sole or significant purpose" was social interaction, which leaves popular gaming platform Roblox exempt for now.
"If the online gameplay is the significant or sole purpose, if that were taken away, would the kids still use that messaging functionality to chat?" eSafety Commissioner Julie Inman Grant told reporters in Canberra on Wednesday.
"Probably not."
A month out from the ban coming into effect, the Albanese government on Wednesday announced the list of platforms it captures has been expanded to include the forum website Reddit and live-streaming platform Kick.
Facebook, Instagram, Snapchat, TikTok, YouTube, Threads and X are among the platforms that will need to take "reasonable steps" to prevent under-16s from holding accounts.
In a National Press Club address last week, Australian Federal Police Commissioner Krissy Barrett said Roblox, whose primary user base is aged five to 13, is being used by "sadistic men" to target young girls.
Ms Inman Grant acknowledged "exemption does not mean a platform is absolutely safe" and eSafety dealt with "sadistic sextortion" across various platforms daily.
"We've got codes and standards and we've used this in our negotiations with Roblox," she said.
"Roblox will be rolling out age assurance technologies."
Discord and Twitch won't be affected by the December 10 ban but Communications Minister Anika Wells said the list would be fluid, meaning more platforms could be added.
Opposition communications spokeswoman Melissa McIntosh accused the government of making policy on the run by adding platforms weeks out from the ban.
"This is all about protecting children from harm," she said.
"It's quite confusing, parents and families still don't know what's going on."
Ms Inman Grant accepted children will try to circumvent the restrictions but expressed confidence in platforms to tackle this.
"We have extensive requirements we place back on the platforms themselves to prevent circumvention, so location-based circumvention in terms of the use of VPNs," she said.
"We know generative AI, a mask or graphics from platforms can be used to spoof AI or age verification systems.
The onus for complying with the ban rests with tech giants to "detect and deactivate or remove" accounts from underage users.
Social media users cannot be forced to provide government-issued ID as the sole method to prove their age online and must be offered other options.
Companies that fail to comply could be fined up to $49.5 million.
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