Australian Business Number (ABN)
A unique 11-digit number identifying a business entity to government and the community.
An Australian Business Number (ABN) is an 11-digit identifier issued by the Australian Business Register (ABR) to businesses operating in Australia. You need an ABN if you carry on a business, regardless of whether you are a sole trader, partnership, company, or trust. It is used on invoices, when dealing with other businesses, and when registering for GST.
If you provide services without an ABN and the payer reasonably expects you should have one, they are required to withhold 47% from your payment (the "no-ABN withholding" rule). Having an ABN allows you to issue proper tax invoices, claim GST credits (if GST-registered), and interact with government agencies efficiently.
You can register for an ABN for free through the ABR website (abr.gov.au). You'll need your TFN and details about your business structure. Be aware that having an ABN doesn't automatically mean you are running a business for tax purposes — the ATO can cancel ABNs if it determines no business activity is being conducted.