Apparently there are several differences
revolving around the Roo's and Emu's number
of toes or lack of same, plus an error
in which the "star," or "gold of the star"
are present, or absent. On one variety
the Roo's eyes are a dot, the other a "line."
Gibbins also mentions that there are other
minor varieties. You really need the Aussi
specialist catalogue to go much further.
From what little I know about Australian stamps
and Australian collectors, I would be surprised
if there is any of their stamps that has
not been examined with such a passion,
that differences have not been found, especially
the pre-2000 issues.
Give them a can of Fosters and a magnifying
glass x20, and they will find something of interest.