Angus and Robertson Limited (A & R) were the publishers of the books of Ion L Idriess (Cornstalk, which published Idriess's first book, Madman's Island in 1927, was an offshoot of A & R). They had their ways of delineating the editions they published, which were often idiosyncratic and inconsistent. They were, after all, about making money, and not catering for (later) book-collectors and bibliophiles. So terms like editions, impressions and printings or reprints were often used interchangeably or inconsistently. Before looking at some specifics, here are some definitions (and some helpful websites that give further information).
Edition: One of a number of printings of the same book, newspaper, etc., issued at different times, and differing from another by alterations, additions, etc. (as distinguished from impression). The Concise Macquarie Dictionary.
Impression is when they print off another load of books using the same set-up. Publishers will make it a new edition when they re-set the type, make corrections, or just sometimes when they feel like.
Andyl in LibraryThing Forum
Reprint: A new impression, without alteration, of any printed work. The Concise Macquarie Dictionary.
First edition: Collectors use the term “first edition” as a short form of “first printing of the first edition”.
http://www.ioba.org/pages/resources/first-edition-primer/
Book collectors generally use the term first edition to mean specifically the first print run of the first edition (aka "first edition, first impression").
https://en.wikipedia.org/wiki/Edition_(book)
See also:
https://rhollick.wordpress.com/2020/05/14/first-printing/
State: The changes during the production of the first printing are called "states".
https://www.biblio.com/book-collecting/basics/how-to-identify-first-editions/
These are mid-print run minor changes, such as binding or colour (see www.idriess.com.au Collections of Idriess works - The national Editions (1939 and 1941) for a discussion of different bindings and colours - states - of these editions).
Usually Ion Idriess books have a date (the year) on the title page and also on the verso (the back of the title page), and often the edition was noted (usually on the title page at least). On the verso of many of his books often further information was given, such as the first publication year, and subsequent editions, impressions, reprints, revisions and special editions (such as the National Edition). Sometimes the verso included not only the years, but the months or even the exact dates of editions, especially when these "editions" (print runs?) were close . For example, the third edition of Gold-Dust and Ashes has on the verso: First Edition . . . March 1933 Second Edition . . . April 1933 Third Edition . . . May 1933; and the second edition of Drums of Mer has: First Edition . . . 30 September 1933 Second Edition . . . 4 October 1933.
So far so good. Except that: are these editions of Gold-Dust and Ashes and Drums of Mer really editions (with changes) or really reprints or impressions? Why does a first (1938) edition of Madman's Island just have the year (on title page and verso), while a 1949 "edition" has on the verso First published 1927 (even though this was very different with major changes) and then has reprinted 1938 (and reprinted for subsequent years)? Why does a second edition of Flynn of the Inland 1932 have on the verso, First Edition 10 March, 1932 Second Edition 16 March, 1932, when no alterations were made between these "editions"? And then the twenty-second edition came out in 1942; but the 1956 Memorial Edition notes that the only new edition was 1951, reprinted in 1955, and prior to that it was first published 1932 and then lists 13 reprints, 1932 to 1949, but only 7 reprints 1932 to 1942? Are these editions or reprints? And if there was a 22nd edition in 1942, what happened to the other editions, or reprints, as reported in the 1956 Memorial edition, as there is a discrepancy of 14 "editions"? There were four 1932 editions of the The Desert Column, but were they really editions or impressions? The third "edition" has First Edition, then, Second Impression, Third Impression, but the fifth "edition" identifies them all as editions! Why? Perhaps the answer is in the definition of Impression by Andyl, above: Just sometimes when they [A & R!] feel like!
Having a year of publication is helpful. But having a year of publication with no advice that this may be a second edition is confusing! So The Great Boomerang was first published in 1941; but the second edition has only the date (1942) on title page and verso - so you would be forgiven for thinking it a first edition (as that is the style for many Idriess first editions). Similarly, The Great Trek was published in 1940, but the second edition again has no notation to that effect and simply the 1941 date - it looks like a first edition but isn't. Later books by Idriess in the 50's and 60's at least specified, for example, First published in 1953. So collecting Idriess can be confusing, and there seems to be no rhyme or reason to some of the edition information that A & R have (or haven't!) included. Caveat Emptor! Thank goodness for sites like this that give so much helpful information in the Bibliography and elsewhere.