Over-sharing by public servants at the Defence Department has emerged as a national security risk, according to Defence bosses.

The departments' 17,500 civilian officials and their uniformed colleagues have been warned of the "security incidents" caused by over-sharing of documents and other material on it internal systems.

Department of Defence logo Credit:Noel M. Towell

The public servants and Defence personnel are being asked to learn the "need-to-share" principle, which is to operate alongside the time-honoured "need-to-know" rule of thumb.

But one workplace union says the crackdown on sharing is more about career-minded senior bureaucrats trying to avoid public disclosures embarrassing them and their political bosses.