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.

Civil liberties groups say using the scanners will make departure security lanes much slower. The department said public reaction to the trial was positive, but admitted it showed screening officers needed training so they could ''exhibit empathy and be able to put themselves in the shoes of passengers who may believe that they are being unduly inconvenienced or mistreated''. The legislation allowing the use of body scanners has been referred to the House of Representatives infrastructure and communications committee, but the government has already announced that the machines will be phased in from July, with a tough ''opt-out-no-fly'' policy at Australia's eight international airports.

''In effect, people are being asked to undergo the equivalent of a strip-search with a bag over their head,'' the president of the Queensland Council for Civil Liberties, Michael Cope, said in a submission to the inquiry. ''The fundamental question is whether the risk of being killed in a terrorist attack, which is considerably less than dying crossing a road or falling off a ladder, justifies strip-searching every person who gets off the plane,'' he said.

''As one privacy expert put it, if you are trying to stop bombers at the airport, it's probably too late.''

Mr Cope said the government's decision to ensure that the scanned image would be a stick figure rather than a person's naked body shape was a ''significant advance''. But it was still denying passengers the choice of a body frisk search, which some might prefer, he said.

The chairman of the Australian Privacy Foundation, Roger Clarke, said the bill before Parliament went beyond allowing the introduction of one specific type of body scanner.

''If they want to zap everybody with different wavelengths of the electromagnetic spectrum, this bill allows them to do it,'' he said.