Historical information
For more information there is a similar envelop with stamps at the Melbourne Museum : https://collections.museumsvictoria.com.au/items/1692627
Significance
There were five issues in the 'Famous Australians' series released as follows: Issue 1, November, 1968; Issue 2, November, 1970; Issue 3, May. 1973; Issue 4, August, 1975; Issue 5, November, 1976.
Physical description
Have a look at the description from Melbourne Museum: https://collections.museumsvictoria.com.au/items/1692627