Maybe this belongs in a Print Ephemera catagory as its definitely not a Railway Parcel Stamp but its closely related to Railway stamps so it may be better listed here.
This stamp apprears to have been issued by Webster Rometch, who operated the bus service ( and had various Post Office delivery contracts) for the Hobart - Huonville-Geeveston route via Ferntree and possibly others at least from the early 1900s. Its overprinted Astor Motors Hobart suggesting Websters was bought out by Astor Motors perhaps.
Interestingly, the stamp design is closely modeled on the Tas Government Railway Parcel stamps 1st - 3d Garret Issues 1917 - 1950 (I am sure someone will correct me if I have my facts slightly amiss here)
I have never see another example of this stamp, and have no knowledge of Astor Motors although Webster Rometch are well know to Post Card and Postal History collectors.
The image of the bus in the central circle might help to date the stamp. Looks definately pre- WW2 to me, possibly 1920s - 1930s.
Can anyone shed any light on the stamp, the Astor Motor company or date this more accuratly ?