It has a lot to do with the tax and rego on cars. In Japan, the older a car gets, the higher the rego and other costs go - it is not impossible to own an older car, just increasingly expensive. It is also true that some cars do come into Oz with low (relatively)milage, however this is not always a good thing as it could indicate that it has spent most of its life with the motor running in a traffic jam or in snow as mentioned.
Having said all of that, don't be a gullible git - if its too good to be true, its not!!