Mulga Queen,
Geoff Kellow in
'The Stamps of Victoria' notes that the 1883 Post Office Act required that any stamp could be used for revenue or postal purposes. Hence it was decided to aim that all stamps would be inscribed 'STAMP DUTY' by 1st January 1885. New issues were inscribed STAMP DUTY and the existing stock of some issues were overprinted in early and late 1885.
I read in paragraph 3 of the
1883 Post Office Act that the definition of "stamp" includes the stamp impressed on any post card, so existing stock of the reply card that you have shown would probably have been overprinted for the same reason.
Geoff Kellow states that archival records show two printings in January 1885 and June 1886. He also shows the three types of overprint orientation.
The three orientations are also described in the Higgins and Gage
World Postal Stationery Catalog, yours being type #2 and with a lower catalogue price (in my 1984 edition) than types #1 and #3.