You answered your own question–they use different radio frequencies. There are no bridging devices for this purpose.
Nothing to do with SmartThings – The company that makes the Z wave chips has assigned different frequencies to different regions and expects them to be used in that fashion. The frequency is set at the time of manufacture and cannot be changed after.
If you can get an Australian frequency Z wave controller that has a cloud service or an IFTTT channel you could communicate cloud to cloud. But that would introduce both complexity and lag.