Ploughman's Lunch first issue
Text reads: This is the first issue of a pamphlet of poems entitled ploughmans lunch is presented to the National Library of Australia in accordance with Section 201 of the Copyright Act 1968. Contributions of poems, graphics drawings and money are eagerly sought by the humble editor. Also Librarians heads on silver plates. (Signed) Gary Oliver 1. January 1974.
Opp. text reads: Special erratum sheet prepared for copy number 155 forwarded to National Library & posterity.
As you can see, the answer is actually very simple; it is because (lucky for me) the poets knew section 201 of the copyright act 1968 which required them to send copies of their magazines to the National Library, Canberra. Hence why the National Library has almost every poetry journal from 1968 onwards that I want to study.
COPYRIGHT ACT 1968 - SECT 201
Delivery of library material to the National Library
(1) The publisher of any library material that is published in Australia and in which copyright subsists under this Act shall, within one month after the publication, cause a copy of the material to be delivered at his or her own expense to the National Library.
(2) The copy of any library material delivered to the National Library in accordance with this section shall be a copy of the whole material (including any illustrations), be finished and coloured, and bound, sewed, stitched or otherwise fastened together, in the same manner as the best copies of that material are published and be on the best paper on which that material is printed.
(3) When any library material is delivered to the National Library in accordance with this section, the National Librarian shall cause a written receipt for the material to be given to the publisher of the material.
(4) This section is not intended to exclude or limit the operation of any law of a State or Territory (whether made before or after the commencement of this Act) that makes provision for or in relation to the delivery to a specified public or other library in or of the State or Territory of copies of library material published in the State or Territory.
(5) In this section:
"illustrations" includes drawings, engravings and photographs.
"library material" means a book, periodical, newspaper, pamphlet, sheet of letter-press, sheet of music, map, plan, chart or table, being a literary, dramatic, musical or artistic work or an edition of such a work, but does not include a second or later edition of any material unless that edition contains additions or alterations in the letter-press or in the illustrations.