Aha! found it through a circuitous route.
I have vague recollections of having seen it once before in 16 years of collecting. The violet ink is a unusual colour for Australia.
The date is what is amazing.
The Wmk identifies it as Sc# 42 1915 and onwards.
1 shilling blue green. ($225 / $30)
This "Timestamp" type postmark became ubiquitous in Western Australian philately, in the 1980's, the early date is the enigma / curiosity.
Sadly, Postmaster, I do not think the image will help you very much.
I spent the whole afternoon searching.
Notes: Faint "PARCEL POST" text
outer ring is segmented by lines indicating time, (usually) the head of an arrow indicates the time it passed the PO staff, pointing at a line,
one can see "9" "10" and "11" in the ring shown.
The lines indicating the quarter hour.
Date appears to be 6th May 1920.
One would be drawn to think, that due to the high value Kangaroos oft being used for Parcels, why this Pmk is not more often met with.
Thought bubble: Perhaps this Pmk is an adjunct to a registration requirement.