During seven weeks of trials at Sydney and Melbourne's international airports, just 57 per cent of the 23,500 volunteers undergoing body scans were cleared immediately to proceed to their flights, according to a Department of Infrastructure and Transport submission to a federal parliamentary inquiry.

Cash, boots with buckles, hairclips, watches, jewellery, studs and zips were the most common causes of the alarm going off in the scanners, which the department has admitted proved slightly slower than the metal detectors now in use.