An ISSN is an 8 digit unique identifier number assigned by the National Library of New Zealand to the title of a periodic publication issued in successive parts, identified by a number and/or a date, linked by a common title, and intended to continue indefinitely. Contact the National Library on 04 474 3074
If you are asked to encode an ISSN or ISBN in a barcode for retail purposes contact GS1 for assistance. There is a protocol for extending the number to a thirteen-digit form. GS1 will do this for you at no cost.
The extended number is encoded in an EAN-13 bar code. Most printers will be able to do this for you when you provide them the number.