Meta, Snapchat, TikTok and Google on Friday appeared before a Senate inquiry considering legislation to impose bigger fines on tech giants failing to comply with Australian law.
The parent company of Facebook and Instagram told the inquiry that it was trying.
"We're doing our best to identify and remove accounts in compliance with the law," Meta's regional director Mia Garlick said.
She said the company had removed 750,000 accounts in Australia, assessed to be belonging to people below the age of 16.
However, she said the company was unsure how many Australian children were still on its sites.
TikTok said it was also unsure.
It had removed 550,000 accounts belonging to teenagers since the ban was imposed, head of public policy Jessika Lofstedt said.
The Senate inquiry will shape legislation in parliament proposing a $99 million fine on tech giants failing to prevent children under 16 from being on their platforms.
Liberal communications spokeswoman Sarah Henderson said the platforms still had questions to answer.
"Meta is fundamentally failing in its responsibilities to keep children safe online," Senator Henderson said.
The harm the platforms have caused Australian teenagers was also canvassed in the hearing with evidence from Wayne Holdsworth whose son Mac committed suicide after being sexually exploited online.
"I don't want another parent in a funeral director's office deciding on what flowers should be on their son or daughter's coffin," Mr Holdsworth said.
Meta earned $60 billion between April and July and it was an issue raised by a mother at the hearing, who too is grieving.
"They are billion-dollar profit centres who have monetised our children and profit from the havoc that they wreak," Emma Mason said.
Her 15-year-old daughter Tilly took her own life after being bullied online. Ms Mason likened the platforms to "social heroin that is killing our children".
A platform noticeably absent from the hearing was X, formerly known as Twitter.
It didn't provide evidence to the inquiry on Friday, but has raised concerns about the bill in a submission.
It said expectations to compel evidence were unrealistic, increased penalties were "disproportionate and inappropriate" and broadening the powers of the eSafety Commissioner could lead to "ordinary Australians" being investigated.
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