There are four formats for GTINs, each with specific purposes. The type of GTIN you use can depend upon on several things, including the size of the item, whether it is retail or non-retail, and where it is being sold.
Company Prefix: This will be allocated to you by GS1 Australia, and will vary in length from six to nine digits
The item reference: This is assigned by you, and we recommend that you begin with zeroes and increase the count sequentially. The numbers have no meaning
Check digit: This is calculated using the first twelve digits of the number. Use this Check Digit Calculator.
GTIN-13:

The GTIN-13 is the most commonly used GTIN for retail trade items. It can also be used for non-retail items like outer cartons, display cartons and more, particularly if these levels of packaging could also be sold to a consumer at point-of-sale.
Retail trade items fact sheet
Non-retail trade items how to guide
GTIN-8:

GTIN-8 allows a smaller barcode to be used on smaller retail trade items. If you believe your product is too small to allow a regular sized barcode, you can apply directly to GS1 Australia for a GTIN-8. Your application will be assessed based on product and packaging size.
How to guide
GTIN-12:
Over the past 15 years, United States and Canada have steadily been updating retail Point-of-Sale systems to be able to accept GTIN-13 and GTIN-8. If you are exporting trade items to these countries, you should confirm with your trading partners if they can scan these numbers. If they cannot, you will need to use a GTIN-12. You can apply for this through GS1 Australia.
Retail trade item how to guide
Non-retail trade item how to guide
GTIN-14:

The GTIN-14 is used on general distribution trade items – that is, items that are not intended for sale to the consumer at Point-of-Sale.
How to guide