The Immigration Department risked shaking public faith in its own work by discovering and revealing the identity of an official who tweeted attacks on the federal government, her lawyers say.
Bosses wrongly sacked Michaela Banerji in 2013 over her anti-government tweets, gagging her free speech even though her comments were anonymous when posted, the High Court has been told.
Ms Banerji's legal clash with the federal government entered another chapter on Wednesday as the High Court was told her former employer discovered she authored the tweets by examining a folder on her desk in 2012.
"Where, as here, anonymity is dissolved by the employer examining a folder of personal material on the anonymous speaker's desk, it is that employer, not the employee, who has threatened, or connected the impugned conduct with, the Australian Public Service's apolitical image," the court was told.
Lawyers Ron Merkel, Christopher Tran and Celia Winnett wrote in a submission that at the time Ms Banerji tweeted attacks on the government, her audience didn't know she was an Immigration bureaucrat.
Twitter users couldn't trace the anonymous comments to a public servant, and the tweets could not have undermined faith in the bureaucracy's impartial role, the lawyers said.
"By their nature, anonymous communications are incapable of producing these effects," they said.
Rules for public servants protecting faith in the bureaucracy's non-partisan role did not require officials to be politically neutral or impartial in their daily personal lives.
Nor did they restrict anonymous criticism of the Australian Public Service, given the importance of free speech about government and politics.
"The APS is not so fragile that its integrity and good reputation cannot withstand anonymous criticism."
If rules stopped public servants from expressing political views or criticising the government in any situation, including in anonymous comments, they would pose an extraordinary intrusion on free speech that did not protect the bureaucracy's reputation for impartiality.
"It would, in practice, prevent APS employees from communicating political views in circumstances with no possible connection to their employment."
Immigration officials had failed to consider the burden on free speech in sacking Ms Banerji, and could have considered other, less punishing responses upon outing her as the author of the tweets, her lawyers said.
"The termination decision is of a kind that burdened free communication on political matters, by requiring a public servant to pay a serious price for political comment."
Ms Banerji's lawyers did not dispute that public servants were restricted from making political comment when it could bear on their role as federal bureaucrats. They said when applied correctly, rules upholding the public service's reputation had a legitimate place.
"This is not to accept that the Commonwealth has any legitimate interest in cleansing APS employees of political opinions or of the ability to express them in ways that do not have a bearing upon the APS as an institution."
The federal government argued last month political expression for public servants was rightly restricted to protect the bureaucracy's impartial role in the interests of responsible and representative government.
In response, Ms Banerji's lawyers asked where the line was drawn when constraining free speech for bureaucrats to protect the public service's non-partisan role, saying not all rules would uphold this reputation.
"A law banning public servants from discussing politics with their families around the dinner table is an example," they said.
Attorney-General Christian Porter intervened in the case after Ms Banerji won an appeal against the federal workplace insurer's refusal to compensate her for the psychological condition that developed after she was sacked over tweets from her pseudonymous Twitter account with the handle @LaLegale.
He removed the government's Federal Court appeal against the finding and sent it to the High Court, flagging the case's potential to undermine the government's policy stopping public servants from expressing their political views on social media.
The federal government is expected to reply to arguments from Ms Banerji's lawyers on December 19.