Details of the hunt by Department of Human Services managers for one its employees who criticised the department in online forums, read more like an FBI crime thriller than routine public service business.

And the Commission's decision has dealt a serious blow to the Australian Public Service's crackdown on the social media use of its employees.

In a damaging outcome to the department, FWC also found it used one of its official social media channels to supply misleading information to the public and that DHS sacked its internal critic after he told the truth about waiting times.

Centrelink worker Daniel Starr was tracked-down and sacked after he went online to contradict the department's "ridiculous assertions" about Centrelink waiting times, blasted some of his colleagues and wrote that he had no idea what senior DHS managers did with their time.

But the department will now have to give the 21-year-veteran his job back in a decision that is a serious blow to the Australian Public Service's policies on the social media activities of its workers.

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The Fair Work Commission found public servants were not bound by their code-of-conduct to show deference to the government while on their own free time.

Commission Vice-President Adam Hatcher found the Public Service Act gave departments no general right to discipline their employees for political speech outside of working hours and such powers would be a "gross intrusion into the non-working lives and rights of public servants."

In April, 2015, Mr Starr, posting on a forum as user "mmmdl" engaged in an online argument with an official DHS account, Flick@HumanServices, about waiting times for youth allowance claims.

Mmmdl accused the department of "ridiculous assertions" and urged other users to contact their MPs about the "utterly disgraceful" situation.

Then the hunt for Mr Starr began, ordered by senior DHS bureaucrats Mark Withnell and Melissa Ryan, Vice President Hatcher noted in his decision.

"I would infer that this (investigation) involved a wholesale trawl through all of "mmmdl's" online posts, since it was discovered from those posts that "mmmdl" claimed that he was approximately 39 years old in January 2015, had been employed at Centrelink for 20 years, lived opposite the Telstra exchange in Corrimal, and would be travelling overseas during late May to early June 2015," Mr Hartcher wrote.

"These claims were matched with the Department's internal data sources, and permitted "mmmdl" to be identified with a high degree of confidence as Mr Starr."

Mr Starr took to the Sportal forum in 2013 to write about cuts to the public service.

"œPart of that "˜understaffing"™ is the fact that, yes, we have our share of utterly useless people that couldn't get a job anywhere else ... I honestly have zero idea what all our managers do, especially the higher managers.

"None at all."