Attorney-General Christian Porter is intervening in a government appeal against a decision finding the Immigration Department unlawfully sacked an official for anonymous tweets, sending the case to the High Court.

The federal government challenge to the Administrative Appeals Tribunal finding could test freedom of speech for public and private sector employees as lawyers spar over former public servant Michaela Banerji's right to post criticisms of asylum seeker policy while working for the department.

Michaela Banerji was dismissed by the Immigration Department in 2013 over anonymous tweets criticising asylum seeker policy. Credit:Karleen Minney

Lawyers for Ms Banerji, who was dismissed over the tweets in 2013, are not expected to oppose the move sending the case to the High Court and will look for a senior legal counsel to represent her when it hears the government's arguments against the appeals tribunal decision.

The Australian Human Rights Commission also intends to enter the case and make submissions about human rights issues it raises including the right to freedom of expression and the right to participate in public affairs, but will not represent Ms Banerji.