The High Court is being asked whether limiting children and young people's political engagement outweighs the harm done by social media platforms, and if it breaches the limit of implied freedom.
The two-day hearing before all seven justices concluded on Wednesday.
Barristers for the federal government outlined seven risks posed by social media platform design features including addictive use, adverse sleep effects and depression.
Anxiety, suicidal ideation, exacerbation of disordered eating and intensification of ADHD symptoms were also risks, barrister Joanna Davidson SC said.
The government maintains the burden on the freedom of political communication was "indirect in nature and modest in extent", barrister Stephen Lloyd SC said.
Mr Lloyd rejected alternatives put forward by Reddit the day before, arguing they did not provide children and young people similar protection to the government's current ban.
If Reddit was to create a platform specifically designed for children, it could be allowed under the ban as it would not include features the Commonwealth has identified as potentially harmful.
"It would be a platform for kids that protects kids," he said.
But in his rebuttal, Reddit's barrister Perry Herzfeld SC argued the current law did not allow for children to have any accounts, including restricted ones.
He said the government's ban produced the "worst possible outcome" because children were robbed of the ability to view safe online content but could still use accounts from adults, which exposed them to harmful content.
Mr Herzfeld told the court the government's ban was "ill-targeted" and "over-broad", rejecting the forum-based platform's inclusion on the list of 10 banned platforms.
The ban which began in December 2025 also includes TikTok, Snapchat and the Meta platforms Facebook, Instagram and Threads.
Kick, Twitch, X and YouTube are also listed, but Discord, Whatsapp and Messenger are not age-restricted.
The Digital Freedom Project's legal counsel Adrian Maroya said if under-16s were protected by the implied freedom of political communication, modes of communication offered by social media were integral.
State and territory solicitors-general have all formally intervened in the case and sided with their federal counterpart.
The Free Speech Union was among other groups permitted to make written submissions.
The High Court has reserved its decision until a later date.
A 16-year-old founding member of The Digital Freedom Project, which launched the legal challenge in 2025, says young people should be able to share their views online - otherwise they would be "spectators in their own world".
"They shouldn't be shut out," Noah Jones said outside court on Tuesday.
Reddit filed its own challenge in 2025.
The government has acknowledged concerns the law might impinge on the rights of children but believes the ends justify the means.
While other countries have subsequently adopted similar bans, the effectiveness of Australia's restrictions have been frequently called into question.
A report by the eSafety Commissioner, which enforces the ban, in July said more than five million Australian accounts had been removed or deactivated.
But four in 10 children aged 10 to 15 remained active on social media three months after the ban began, down from 52 per cent.
Labor is pushing to pass its new digital duty of care bill, which would place the legal responsibility on tech platforms to proactively secure their systems rather than relying on users to report harms.
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