You would need to contact the call center to find out 1) if this boot can be shipped to Australia 2) what the current postal rate is for shipping to there.
There are numerous factors of how to order boots, as a general rule you would order 1/2 size smaller than your regular street shoe size in the US market. In the Australian sizing you would order the same size as your street shoe unless otherwise indicated that a boot may run big or small then order accordingly. With Lucchese I found that sometimes the boots ran exact to street shoe sizing since most were made by hand and some sizes can be ordered with a measurement depending on where you go. I would ask the agent when you call to verify that the sizing of the boot if it ran exact to street shoe size or if it was standard. If it is exact to street shoe size then order 1/2 size larger than street shoe size since the difference between US and AU sizing is 1/2 size.