The High Court heard rules that brought on the 2013 dismissal of former Immigration Department worker Michaela Banerji intruded unacceptably on free speech.

Lawyer Ron Merkel told judges the restrictions on public servants' political expression overreached when applied to anonymous comments. Ms Banerji, who used a pseudonym to tweet criticisms of the government to her 700 Twitter followers, wasn't identified until her own department outed her in 2012.

The public needed to know about the non-confidential workings of the bureaucracy and restrictions to free speech for bureaucrats impeded the flow of this information, the court heard.

Rules that brought on Ms Banerji's sacking applied too broadly by letting public service employers sanction staff for creating the risk of harm to the bureaucracy's apolitical reputation in the case they were later identified, Mr Merkel said.

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

Mr Merkel agreed with federal government lawyers the constitution recognised the public service's importance, but warned this raised the value of their free political expression.

"This is one of the problems of the overreach, because it effectively shuts off critical comment independently of the harm that it might cause," he said.

"An over-broad restraint on communication by public servants poses a particular risk to the system of representative and responsible government because it represents a risk of stifling the voices of those particularly aware of how the public service and the executive are operating.

"We say that that does not result in no restraint but we say that the court should be particularly astute to scrutinise those restraints."

Communication between ministers, bureaucrats and the public was as necessary to democratic government as communication between the people and their elected MPs, Mr Merkel told judges.

"We say that the class of persons burdened and the breadth of the burden presents a risk to the efficacy of electoral accountability for the exercise of legislative and executive power," he said.

Solicitor-General Stephen Donaghue on Wednesday argued the restrictions on free speech for public servants may be more readily justified than for other groups, because they promoted an apolitical, impartial bureaucracy needed for democratic government to work.

Freedom of speech implied in Australia's constitution accommodated the need for an apolitical public service, he said.

Solicitor-General for Western Australia, Joshua Thomson, on Thursday said rules requiring public servants to uphold the bureaucracy's apolitical role only prohibited them from expressing a preference for a political position, not stating its advantages and drawbacks.

South Australian solicitor-general, Chris Bleby, said the restrictions included a reviewable process letting decision-makers apply them by considering a public servant's role, level of seniority, and the connection their comments had with their work.

"A key feature of responsible government is accountability. So an attempt at anonymity can only be an attempt to avoid that, to avoid accountability and, therefore, undermine responsible government," he said.

High Court judges heard the case after the Administrative Appeals Tribunal last year found Ms Banerji's dismissal unacceptably trespassed on her freedom of speech. Attorney-General Christian Porter sent the case to the court in a move showing its potential to weaken the government's limits to political commentary among bureaucrats.

The High Court reserved its decision.