ISBN stands for International Standard Book Number, and you’ll require one for your book.
For one thing, it generates the 13-digit barcode that goes on the back cover. Once your book has an ISBN, that assignment lasts forever. The ISBN enables booksellers all over the world to access the rest of the meta-data for the book – such as author bio, publisher details, subtitle, edition, dimensions, weight, etc. – and it spreads throughout all databases that hold book information.
This international system of identifying books requires you to have a separate ISBN for each edition and format of your book. So if you print a paperback and also create an e-book, that’s two different editions.
You can obtain an ISBN in New Zealand through the National Library. You will, in any case, need to contact the National Library to register a Cataloguing-in-Print (CiP) record for your book, whether it’s electronic or print or both. This ensures that they know your book exists. (They will ask you for two ‘legal deposit’ copies of your book, too, so that it becomes part of the nation’s literary heritage.)