The independent federal body is backing former Immigration Department worker Michaela Banerji in arguing her employer was wrong to sack her in 2013 for anonymous anti-government tweets.

Lawyers Kristina Stern and David Hume, for the Australian Human Rights Commission, said the role of public servants gave their free speech a special value.

"Political communication by public servants will often be crucial to the vitality and accountability of the system of responsible government," they said.

When speaking their views, bureaucrats had expertise and experience that may be critical in holding the government to account.

Burdens on free speech for public servants were as significant as those on parliamentarians in their potential effect on responsible government, Ms Stern and Mr Hume said.

Arguing for Attorney-General Christian Porter, the federal government's solicitor-general Stephen Donaghue has said the freedom of speech implied in Australia's constitution accommodated the need for an apolitical public service.

These combined to suggest that "burdens on political communication by public servants may be more readily justified than similar burdens on other groups," he said.

The human rights commission has taken the opposite view, saying "political speech by public servants is particularly important".

"Burdens on political speech by public servants are less likely to be justified than other burdens on political speech," Ms Stern and Mr Hume said.

The commission supported Ms Banerji's case in a written submission to the court on Wednesday, but is not a party in her legal battle with the Attorney-General.

It has involved itself because the court's finding could limit, or expand, free speech and the right to participate in public affairs.

Ms Stern and Mr Hume told the court previous governments setting rules for public servant conduct had recognised bureaucrats should take part in Australia's political life and could improve understanding of policies.

Guidelines in 1979 said it would be wrong to deprive political debate of the talent, expertise and experience of public servants simply because of their roles in government.

Limits set by 1995 rules hinged on the seniority of the bureaucrat, how close they were to policy commented on, the harshness of the criticism and whether it disrupted the workplace, the human rights commission said.

Mr Porter intervened in the case when Ms Banerji won an appeal against the federal workplace insurer after it refused to compensate her for the psychological condition that developed following her sacking.

The Administrative Appeals Tribunal in April found guidelines stopping bureaucrats from publicly criticising the government should not apply to anonymous comments.

Ms Banerji should not have been dismissed because the decision intruded impermissibly on her free speech, tribunal deputy president Gary Humphries and member Bernard Hughson said.

The Attorney-General sent the government's appeal against the finding to the High Court, flagging the case's potential to undermine rules stopping public servants from expressing their political views on social media.

Ms Banerji was working in the Immigration Department, which recently became Home Affairs, when co-workers learnt she was behind 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".

The federal government is expected to reply to arguments from Ms Banerji's lawyers on December 19.