Australian defence officials have admitted their personnel were responsible for a bomb scare which sparked a major security operation at Sydney Airport this month.

The incident occurred when two inert improvised explosive device (IED) training aids were left inadvertently in the back of a vehicle returned to the airport office of a hire car company, following an unspecified defence training activity.

"The devices were discovered by hire car staff when the car was being cleaned at Sydney Airport," defence said in a "hot issues brief" published on its website.

Australian Federal Police and NSW police were called out and a section of the domestic airport car park was cordoned off while the devices were investigated. No flights were affected.

The incident started about 1pm on July 4 with the bomb squad deeming the items harmless about 3pm.

Defence said both devices were marked "inert" and posed no danger to the public.

- AAP