The book started life as a fund-raiser in 1980 by the Australian Plant Study group (I think Victoria).
It went through a reprint every year in the 80’s. That’s the copy you see on Amazon. There are also soft backed editions. Abe Books is a good place to look as that price on Amazon is not real.
I am not sure when the edition by Natalie Peate, Gwenda Macdonald, Alice Talbot came out. I thought it was updated in the early1990 by Gwenda MacDonald. I see the Booktopia one is 2008. The authors all belong to the Australian Plant Study Group.
I would go for the latest edition you can find (or afford) By way of example my book (1986 edition) has 2300 plants the 2008 has 3000.