Nigel is correct, there was only one die used for the green 1½d. This denomination was originally issued in brown shades (black brown first, but the cancellation was difficult to see so they made it a lighter brown.) To conform with UPU requirements for international mail, the colour was changed to green in Mar, 1923, being the foreign newspaper rate. Due to a rate change from Oct 1923 it became the foreign postcard rate, for which the UPU recommended red. It took them until May 1924 though to change the colour to red.
Hi cdnum The others are correct there is only one die, but there are lots of different varieties listed in the Australian Commonwealth Specialists Catalogue. It is a real mine field. I only just bought a copy, and it is going to take me a long time to work it out. However there is one blessing, it only came in one watermark type and the perf is 14 the watermark is a crown over the A. Regards Horamakhet