I don't the mark is postal. The top arc is virtually non-existent by in the centre it says "BY" and "HARRINGTON"in the bottom arc.
This is what old Australian number plates look like
At left is a vertical VIC for Victoria, New South Wales had NSW, Queensland had QLD, South Australia had SA and Western Australia had WA. I don't know what Tasmania had, but it is a beach scene and while Tassie has some lovely beaches its just too cold there.
It is hard to tell if it three or two letters.