Hello John and Beren. I do not think that it is a commercial aircraft from Europe or elswhere as Perth is not on this kind of high altitude traffic route unlike Sydney or Melbourne. If it isnt landing or leaving they just dont come here!
Wonderful isolation!
You are right about the north/south travell route Beren, I just checked my compass (didnt think of that
) which I think tends to uphold my thoughts that it may be a supply aircraft for one of the bases in antartica.