The union representing federal public servants has lodged an industrial dispute over the Department of Prime Minister and Cabinet's new social media policy.

The new guidelines prevent public servants from posting comments critical of the Federal Government and include a clause instructing employees to report colleagues who break the code of conduct.

It comes after a case involving an Immigration Department employee who was sacked after posting comments critical of the Government's asylum seeker policies.

Michaela Banerji used the twitter handle @LaLegale to post comments to followers about politicians and immigration policies.

Community and Public Sector Union national secretary Nadine Flood says the latest changes are draconian and come with no consultation.

"We're talking about the public service not a police state," she said.

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"We believe that public sector workers should have the same rights as other Australians to engage in online debate particularly when it is in their own time and they're not saying they speak for government.

"This policy could mean a public servant breaches the rules if they go to a political rally on the weekend and someone puts their photo on Facebook. It's just over the top."

The union says the failure to consult with staff and unions over the changes is a clear breach of the enterprise agreement.

Ms Flood says many public servants are concerned about the reach of the guidelines.

"This is a genuine industrial issue for workers who are worried that they might face the sack for some pretty innocuous behaviour that is the same stuff that every other Australian might do on social media on the weekend," she said.

The union wants to sit down with the department to resolve the matter, but says it is willing to take the dispute to the Fair Work Commission if necessary.