Australian Customs and Border Protection Service's 5000 workers were told on Thursday morning to be on the lookout for suspicious persons or vehicles trying to sneak into the agency's sites and officers were urged to stay on the alert when they were away from work.

The agency's acting Chief Security Officer Peter Lovett said in his all-staff bulletin that there had been no direct threats to his workers but that they must "must maintain security awareness and remain vigilant to unusual or suspicious activities."

Mr Lovett told uniformed officers to put a jacket or top over their official garments to avoid making targets of themselves while they stepped out for a sandwich or cigarette break.

"Until otherwise notified, officers are authorised to wear a plain jacket or similar item of non-uniform to conceal uniform shirts or insignia if they need to leave ACBPS premises during lunch, or to and from the workplace," the senior official wrote.

"These are of course to be removed immediately when arriving at work."

Officers were also warned to "cease wearing visible uniform components to or from the workplace" and to "avoid being on their own in public places when uniformed."

Official identification badges and lanyards are to be put away when officers are not on duty and Customs staff are to be aware of the threat of "tailgating" into official premises.

"Actively but politely challenge, and if necessary report, anyone tailgating your entry to ACBPS facilities," Mr Lovett wrote.

"This includes vehicles attempting to enter ACBPS car parks, individuals attempting to enter ACBPS buildings or classified areas through an access controlled door, people on ACBPS premises who are not wearing identification or whose identification is not clearly visible, and people whose identify you may not recognise or whose activities are unclear."