ABN (Australian Business Number)
An ABN is the unique 11-digit number that identifies your business to the ATO and your clients. Required on every invoice in construction.
Ask Chalkline about this →An ABN (Australian Business Number) is a unique 11-digit number issued by the Australian Business Register (ABR) that identifies a business entity to the Australian Tax Office and to other businesses. Every contractor and sole trader operating in the Australian construction industry needs one.
Your ABN must appear on every quote, invoice, and contract you issue. A builder or principal contractor who receives an invoice without an ABN is legally required to withhold 47% of the payment under the PAYG withholding rules and remit it to the ATO before paying the contractor the balance.
Registration is free at abr.gov.au and is usually instant. You must be carrying on a business in Australia to be entitled to an ABN. Employees cannot hold one for the work they do as employees, which is the core distinction in sham contracting cases.
Also known as: Australian Business Number
Category: Tax and business registration
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Last updated: 2026-05-10. Verified: 2026-05-10. Quarterly review for currency.