The sacking of a public servant who sent anonymous tweets criticising the government seven years ago is sending a chill across the bureaucracy.
The full bench of the High Court on Wednesday unanimously found Michaela Banerji breached the public service code of conduct and couldn't claim compensation arising from her 2013 dismissal.
It ruled that while the code of conduct did impinge on the implied freedom of political communication, that was not the same thing as a right to free speech and was necessary to maintain confidence in an apolitical public service.
Ms Banerji tearfully expressed disappointment in the ruling.
"The only advantage of this case ... was to affirm the role of this freedom of speech for public servants and we've failed," she told reporters outside the court.
"It's not just a loss for me, it's a loss for all of us and I'm very, very, very sorry."
The public servants' union said the decision had serious implications for almost two million Australians working for federal, state and local government.
Ms Banerji was fired in 2013 from the then Department of Immigration and Border Protection after it found she was behind an anonymous Twitter account critical of the agency and government policy.
Using the name "@LaLegale", she posted thousands of tweets on topics including offshore processing and Australia's international obligations to refugees.
Following complaints by departmental staff, an investigation was launched into whether Ms Banerji breached the public service code of conduct.
When she received her termination notice in September 2013 she suffered post-traumatic stress disorder and a month later lodged a workers' compensation claim.
The claim was refused and Ms Banerji took it to the Administrative Appeals Tribunal, which ruled in her favour.
As well, the tribunal found the public service's rules around the use of social media and making public comment "unacceptably trespassed on the implied freedom of political communication".
However, the High Court unanimously said the tribunal erred in this interpretation.
In reasons written by a majority, the court said it was unfortunate the AAT appeal was framed in terms that suggested the implied freedom of political communication was a personal right such as US First Amendment or those included in the Canadian bill of rights.
"As has been emphasised by this court repeatedly ... the implied freedom of political communication is not a personal right of free speech," Chief Justice Susan Kiefel and Justices Virginia Bell, Patrick Keane and Geoffrey Nettle wrote.
Rather than allowing people to say anything they like, that implied freedom only restricts legislative powers.
"The provisions (of the Public Service Act) are targeted. They do not apply to the public at large," Justice Michelle Gordon wrote in separate reasons.
The court found the laws were consistent with and important for maintaining an apolitical public service.
Allowing public servants to harshly criticise government policy or members of parliament, even anonymously, could seriously undermine public confidence in the bureaucracy, it ruled.
Justice James Edelman noted that until the 1970s there was a total ban on public servants expressing political opinions.
"The code that now regulates their behaviour no longer turns public servants into lonely ghosts," he wrote.
"But, properly interpreted, it still casts a powerful chill over political communication."
Community and Public Sector Union national secretary Nadine Flood said in the years since Ms Banerji's sacking, the rules governing public servants using social media had become even more draconian.
"'The CPSU has always defended the rights of public servants to participate in our democracy like everyone else can," she said.
"The Morrison government needs to demonstrate that it prioritises democratic rights, with a social media policy that reflects the real world."
Australian Associated Press