Public Service Commissioner Stephen Sedgwick confirmed the legal quirk last month, saying he was unable to intervene in a dispute between Customs chief Michael Pezzullo and Canberra Times reporter Noel Towell.

The decision highlights a broader immunity enjoyed by public servants in Parliament, such as the freedom to ignore their usual duty to "maintain appropriate confidentiality about dealings [they have] with any minister or minister's member of staff".

Parliamentary privilege also protects bureaucrats who perform appallingly in front of committees, such as when they regularly get facts wrong.

Under a 1988 Senate order, officials cannot be disadvantaged by their dud appearances: for example, they must not be denied bonuses or promotions on the basis of their performance in Parliament.

The spat between Mr Pezzullo and Towell was sparked in May, when Immigration Minister Scott Morrison chose to use a newspaper to announce the creation of the Australian Border Force.

Mr Pezzullo alerted his staff to the article and to a speech the minister was to give later that day, saying: "If you haven't yet read the minister's opinion in The Australian, I encourage you to do so."

Fairfax Media published a report by Towell that morning, which said the Customs boss told his staff "to read a newspaper for details of the government's plans for sweeping changes to their workplace".

An angry Pezzullo later told a Senate estimates hearing that the article was "not even remotely" accurate, and described Towell as a "bottom feeder" who should get "a real job".

Under the Public Service Act's code of conduct, bureaucrats must "treat everyone with respect and courtesy" in relation to their work, while agency heads had an extra duty to promote these values and ethics. Public servants who breach the code can be reprimanded, fined, demoted or even sacked.

However, Mr Sedgwick said parliamentary privilege preventing him from investigating Mr Pezzullo's conduct.

The Customs chief told The Canberra Times Towell's article had wilfully misrepresented him. "It would lead any reasonable reader to conclude that I had told my staff to read about the agency's future, and theirs, in the morning media. I did no such thing."

The bible of parliamentary rules, Odgers' Australian Senate Practice, says the immunity enjoyed by committee witnesses is near limitless: "the infliction of any penalty ... in consequence of their giving evidence may be treated as a contempt [of Parliament]".