A surprise trial of controversial "millimetre wave" body scanners is taking place at Sydney International Airport from today until August 19.

The government says passengers will be given the option of standard screening or passing through the new scanner.

Federal Transport Minister Anthony Albanese said the technology was "perfectly safe" and was the first person through the scanner when it was switched on.

The millimetre-waves are about 10,000 times weaker than a mobile phone signal, and considered much safer than the backscatter x-ray scanners that are used at some U.S. airports.

There is not scientific consensus over the safety of the scanners, though. One study by Los Alamos National Labs argued that standards had only been established for radio frequency exposure up to 300 gigahertz, and millimetre wave scanners were above this range.

Even though the strength of the waves is extremely low, the close-to-terahertz radiation used by the scanners could damage DNA in cells.

This is the second time the scanners have been tested in Australian airports. They were previously trialled in Sydney, Melbourne and Adelaide airports in October/November 2008, with over 70,000 passengers volunteering to be "imaged" by the machines. The government did not say why it needed to re-test the machines following the initial trial.

What the Scanners Will Show

The scanners detect metal and non-metal items under clothing, and then pin-points where the item is hidden using a generic human outline (not the "naked" images generated by scanners in the U.S..)

The government says the machines will not store any imagery of passengers (however, the U.S. government also made this assurance, before saved passenger images were leaked to a tech blog by a whistleblower.)

Passengers not wanting to try the body scanner will proceed through standard screening procedures.

Melbourne Airport will also hold a trial of the scanners, before they are progressively rolled out at Australia's international airports.

The government gave no warning of the introduction of the millimetre wave scanners, and has instead been focusing on the introduction of backscatter x-ray scanners to be used only on incoming passengers suspected of concealing drugs internally.

Another type of scanner that emits no radiation at all, but simply senses emissions from the human body was recently tested at Sydney Airport.