From time-to-time the Universal Postal Union members would meet and agree policies to be adopted until the next meeting.
One of them was to set minimum charges to apply for the recovery of postage due.
From 1 January, 1922 to 1 October, 1925 Australia adopted a minimum of 1½d for unpaid or underpaid letters from within the British Empire. A stamp of this value was not issued until February, 1925.
For the period from February, 1925 to 1 October, 1925, this value could be used on its own for recovery of this taxation calculation.
The post card on the first page is one of just four from this period that I have seen. All originated in the UK, all have the 1½d adhesive pen-cancelled.
I am unclear as to why this value was continued after 1 October, 1925 but further printings were made in 1926 and 1929, usage continued into the late 1950s.
Here are my collection pages.