# Australian Business Number Key: `identity.government.abn` 11-digit identifier for Australian businesses, issued by the Australian Business Register. May appear with or without spaces. Page: Samples: `51 824 753 556`, `53004085616`, `91 345 678 901`, `88 765 432 109` ## Background ### Origin The ABN system was set up by the A New Tax System (Australian Business Number) Act 1999, Act No. 84 of 1999.[[1]](https://www.legislation.gov.au/C2004A00467/2026-06-30/2026-06-30/text/original/epub/OEBPS/document_1/document_1.html) The Act received the Royal Assent on 8 July 1999.[[1]](https://www.legislation.gov.au/C2004A00467/2026-06-30/2026-06-30/text/original/epub/OEBPS/document_1/document_1.html) The Act's main object is to make it easier for businesses to conduct their dealings with the Australian Government.[[1]](https://www.legislation.gov.au/C2004A00467/2026-06-30/2026-06-30/text/original/epub/OEBPS/document_1/document_1.html) It does this by establishing a system for registering businesses and issuing them with unique identifying numbers so that they can identify themselves reliably.[[1]](https://www.legislation.gov.au/C2004A00467/2026-06-30/2026-06-30/text/original/epub/OEBPS/document_1/document_1.html) The Act also aims to reduce the number of government registration and reporting requirements by making the system available to State, Territory and local government regulatory bodies.[[1]](https://www.legislation.gov.au/C2004A00467/2026-06-30/2026-06-30/text/original/epub/OEBPS/document_1/document_1.html) The ABN was introduced on 1 July 2000 as part of a major tax reform.[[2]](https://en.wikipedia.org/wiki/Australian_Business_Number) That tax reform included the introduction of a GST.[[2]](https://en.wikipedia.org/wiki/Australian_Business_Number) ### Use The ABN is an 11 digit identifier issued to all entities registered in the Australian Business Register.[[3]](https://abr.business.gov.au/Help/AbnFormat) It is structured as a 9 digit identifier with two leading check digits.[[3]](https://abr.business.gov.au/Help/AbnFormat) The leading check digits are derived using a modulus 89 calculation.[[3]](https://abr.business.gov.au/Help/AbnFormat) To verify an ABN, subtract 1 from its first digit, multiply each of the 11 digits by the weights 10, 1, 3, 5, 7, 9, 11, 13, 15, 17 and 19, and sum the products.[[3]](https://abr.business.gov.au/Help/AbnFormat) If that sum leaves a remainder of zero when divided by 89, the number is a valid ABN.[[3]](https://abr.business.gov.au/Help/AbnFormat) The ABR's own worked example, 51 824 753 556, is written in groups of two, three, three and three digits.[[3]](https://abr.business.gov.au/Help/AbnFormat) Strip spaces before comparing or joining ABNs, since the same number can arrive grouped or as a bare run of 11 digits. (Meridian's view) Run the modulus 89 check as well as a length check: a well-formed 11-digit string can still be a mistyped number. (Meridian's view) The last nine digits of a company's ABN usually comprise its Australian Company Number (ACN).[[4]](https://en.wikipedia.org/wiki/Australian_Company_Number) The last 9 digits of an ABN do not always comply with the ASIC ACN algorithm.[[4]](https://en.wikipedia.org/wiki/Australian_Company_Number) The Australian Taxation Office's own ABN, 51 824 753 556, is given as an example of this.[[4]](https://en.wikipedia.org/wiki/Australian_Company_Number) Do not assume a nine-digit tail is a valid ACN, and do not derive one identifier from the other without checking each against its own register. (Meridian's view) ### Where you'll meet it The Australian Business Register publishes the ABN Bulk Extract on data.gov.au, with each entity's ABN, ABN status, entity type and ACN/ARBN among its attributes.[[5]](https://data.gov.au/data/dataset/abn-bulk-extract) ## Sources 1. [A New Tax System (Australian Business Number) Act 1999](https://www.legislation.gov.au/C2004A00467/2026-06-30/2026-06-30/text/original/epub/OEBPS/document_1/document_1.html) — documentation 2. [Australian Business Number](https://en.wikipedia.org/wiki/Australian_Business_Number) — secondary source 3. [Format of the ABN \| ABN Lookup](https://abr.business.gov.au/Help/AbnFormat) — documentation 4. [Australian Company Number](https://en.wikipedia.org/wiki/Australian_Company_Number) — secondary source 5. [ABN Bulk Extract - Dataset - Data.gov.au](https://data.gov.au/data/dataset/abn-bulk-extract) — dataset