Gossip, including comments made online, can undermine the public's faith in bureaucracy, the public service commissioner says.

Loose gossip about from public servants online can undermine public's faith in the bureaucracy.

A senior federal official has warned public servants to vet carefully what they say online, lest their "gossip" undermine the public's faith in the bureaucracy.

Public Service Commissioner Stephen Sedgwick used the latest State of the Service Report to defend his cautious approach to websites such as Facebook and Twitter.

Mr Sedgwick's agency advised public servants earlier this year on how to make comments and engage in discussions online.

Its guidance said public servants had the same right to freedom of expression as other citizens, but that right was tempered by the need to maintain "an impartial and effective public service in which the community can have confidence".

It advised staff to avoid making "harsh or extreme" criticisms of the government, parliamentarians or a political party's policies, in case the comments raised concerns about their impartiality.

This applied to public servants whether they were at work or home, and even when they made comments anonymously.

Mr Sedgwick acknowledged in this week's report that some people had regarded the advice "as excessively cautious, and worse".

"As commissioner, I make no apology for this. There is much at stake here – including careers if an individual's misjudgement has the effect of impugning their integrity or impartiality."

He said trust in the Australian Public Service would "quickly erode if ministers, shadow ministers, members of Parliament and senators, or the public, form the view that their confidences risk being broken through public gossip, including on the internet, about matters they have a right to expect should remain private".

The State of the Service Report shows only a tiny proportion of employees were investigated in 2011-12 for allegedly leaking information or for behaving improperly outside work hours.

Twenty-four staff were investigated for leaking and just eight were deemed guilty – or one in every 21,000 public servants.

Only 23 staff were investigated for poor behaviour outside work, of whom 17 were found to have breached the APS code of conduct.

The report also says very few public servants (8 per cent) have full access to social-media websites while at work, while almost half (48 per cent) say they have no access.

Just over half of federal government agencies have advised their employees on how to present themselves online when they are using the internet for personal and non-work purposes.