First you need to be able to create the numbers to identify your products. These are called Global Trade Item Numbers (GTINs). They are put into barcodes so that they can be automatically read by scanning. Numbers and bar codes are different things and should be thought of separately to avoid confusion because the same number can be encoded in a variety of bar codes, each of which has a distinct name.
To be allocated GTINs that you can turn into bar codes to put on your products, you need to be a member of GS1 NZ.
Once you are a member you will have access to your own ProductVault, a database on which you can record details of your products. ProductVault will automatically create a GTIN for each product you enter and will lead you to a barcode generator from which you can download an electronic file containing an image of the barcode that you can pass to your printer for use on your package or label.
Alternatively there is a method of manually creating GTINs that is explained in the literature you will receive on becoming a GS1 member.
GS1 prefers and recommends the use of ProductVault. It eliminates the possibility of error or duplication.