The decision on Monday will redirect scrutiny to the Immigration Department's dismissal of Michaela Banerji for tweeting criticisms of detention policies, and challenges Australian Public Service rules stopping public servants from expressing their political views on social media.

Ms Banerji took the government to the Administrative Appeals Tribunal after federal workplace insurer Comcare refused to compensate her for the psychological condition that developed after she was sacked in 2013 over tweets from a pseudonymous Twitter account.

The tribunal overturned Comcare's decision and found she suffered depression and anxiety that could be classed an injury under federal compensation laws.

Ms Banerji was working in the Immigration Department when co-workers learnt she was behind the tweets railing against the government's treatment of asylum seekers.

She lost a high-profile attempt to stop her dismissal in the Federal Circuit Court in 2013, a decision seen as likely to curtail other bureaucrats' use of social media when judge Warwick Neville found Australians had no "unfettered implied right (or freedom) of political expression".

In a case that Ms Banerji's lawyer Allan Anforth from Canberra Chambers said could have implications for other public and private sector employees, the AAT said Comcare's refusal was based on a dismissal that was unlawful because it intruded on her right to free political expression.

Her tweets, made from the Twitter handle @LaLegale, were anonymous and did not disclose confidential departmental information, but an internal investigation in 2012 found she had breached the code of conduct for government employees.

In a submission to the tribunal, Mr Anforth said the tweets were posted from her own phone and, in most cases, outside work hours.

The appeals tribunal found the Immigration Department itself had identified Ms Banerji after she posted anonymously, and said guidelines stopping public servants from publicly criticising the government should not be applied to anonymous comments.

"A comment made anonymously cannot rationally be used to draw conclusions about the professionalism or impartiality of the public service," it said.

"Such conclusions might conceivably be open if the comments were explicitly attributed to, say, an unnamed public servant, but that hypothetical situation does not apply to Ms Banerji."

The tribunal found Ms Banerji appeared to have taken care not to have used information which could only have been in her possession as an Immigration employee.

It lashed the government decision to sack her, saying it "impermissibly trespassed upon her implied freedom of political communication", and "with a law only weakly and imperfectly serving a legitimate public interest".

"The burden of the code on Ms Banerji’s freedom was indeed heavy – the exercise of the freedom cost her her employment.

"In our opinion, there is no significant justification available to the employer here for the law which exacted that cost."

Comcare is considering the tribunal's decision. The findings could be appealed in the full Federal Court.

Mr Anforth said under legislation, due to her age Ms Banerji could receive compensation for medical matters but not for incapacity.

The AAT's decision comes after the government released tough new social media rules that may put its public servants in breach if they criticise policies by "liking" posts on Facebook or Twitter or by sharing negative information or comments in private emails.

New Australian Public Service rules warn that public servants don't have unlimited rights of free speech, ban anonymous posts criticising the government, including those written with a pseudonym, and remind bureaucrats they can be traced through their digital footprint or via a "dob-in" to their department.

The Home Affairs Department said the case was not a challenge to the decision dismissing Ms Banerji from her employment by the former Department of Immigration and Citizenship, while Australian Public Service Commissioner John Lloyd would not comment on the decision but said "anonymity is not as guaranteed as in the past.”

Associate Professor Louise Thornthwaite, from Macquarie University's Faculty of Business and Economics, said the finding was another example of a growing legal recognition that social media was a part of people's lives and there were situations people could expect to be private.

Mr Anforth said the tribunal's decision freed public servants to fully participate in the community, provided they didn't comment on politics in their official capacity.

Community and Public Sector Union national secretary Nadine Flood said the tribunal decision was significant for public servants, although it was subject to appeal.

“This decision offers a far more reasonable interpretation of the APS Code of Conduct, making it clear that an anonymous criticism on social media from a Commonwealth worker is no different to if that criticism was made by any other citizen,” she said.