The ABN is a unique 11 digit identifying number that businesses use when dealing with other businesses. For example, you generally need to put your ABN on your invoices, or other documents relating to sales that you make. If you don't, other businesses may withhold 46.5% from any payment to you.

You also need an ABN in certain dealings with the Tax Office and other areas of government.

Registering for an ABN is not compulsory, but you will need one to register for the GST. Your ABN allows you to:

• facilitate a single Business Activity Statement
• confirm your business identity to others when ordering and invoicing
• avoid PAYG tax on payments you receive
• claim GST credits
• claim energy grants credits
• obtain an Australian domain name.