All true points. I referred to the Italian House
Sparrow as a subspecies: Passer domesticus italiae or P.
hispaniolensis x italiae, thereby including the possibility of them
as a hybrid form. I think it is just one of those taxonomic vague in
betweens. These odd birds seen would most likely be an independent
variation from the wild birds, that exhibits the same plumage features, whether
by the same or a different genetic method who knows. The paper by Nicola
Clark is good but does not contain much that you (Mark) wouldn't know. It
highlighted the other differences as to why it wasn't a Tree Sparrow, mainly
in that the Tree Sparrow has two white wing bars and the House Sparrow has
one. Also the Tree Sparrow's overall colouring is a much richer chestnut than
any male House Sparrow.
As for the Tree Sparrow arriving in Australia
naturally, well maybe and with ship assistance more likely, but it hasn't
happened yet, they don't fly very well. Among the obvious field visible
differences between the two species, at least among the many I measured, the
Tree Sparrow's primaries are shorter than the House Sparrow's primaries.
What is an obvious field visible difference, is not the length itself but
the shape, the Tree Sparrow's wings are much rounder, giving them a quite
different appearance at a distance.
I studied the two species full time for nearly a
year as part of my honours thesis.