This discussion has been up before, but don't forget the proposed scanners for Australia are the millimetre-wave scanners, not the backscatter xray scanners used in some other countries. There's a significant difference in the technology from a health perspective. Millimetre waves do not initiate cancer, for example (unlike xrays).
But privacy concerns are serious. These devices can present a reasonably clear image through clothing. Software is available which converts this to a generic human outline, but this software isn't widely used and abuses at the TSA have already demonstrated that privacy concerns are quite valid.
Even worse still, and bound to be completely ignored by the Government, is the fact that their effectiveness is under a cloud. Earlier this year, an undercover TSA agent was
repeatedly able to get past these scanners carrying a concealed handgun in covert testing at Dallas-Fort Worth! The TSA and Governments maintain that these scanners are effective, even though not a solitary terrorist has ever been detected by one. So what they base their data on is somewhat of a mystery to us all.