There are different types of walnuts and each of them are native to various areas of the world. Black walnuts for example are native to the United States whereas English walnuts are native to Persia. Black walnuts tend to have a very strong flavor with an extremely hard shell.
Most of the commercially-produced walnuts are various hybrids of English walnuts. Although it is indeed true that Australia imports and sells millions of walnuts each year, there are no varieties that are native to this land. Black and English walnuts are the two most common kinds and they are from Persia and the United States.