ABN - Apply for an ABN (Australian Business Number)

An ABN (Australian Business Number) is an 11 digit identifier that makes it easier for businesses and all levels of government to interact. You do not have to have an ABN, but having one will avoid having amounts withheld from payments to you; and make it easier to register for GST and other business tax registrations such as PAYG withholding. You can apply for an ABN (Australian Business Number) or access and update your ABN details online.

http://australia.gov.au/service/abn-apply-for-an-abn-australian-business-number

ABN - Lookup an Australian Business Number

ABN Lookup provides access to the publicly available information provided by businesses when they register for an Australian Business Number (ABN).

http://australia.gov.au/service/abn-lookup-an-australian-business-number

ABN essentials

Australian Taxation Office information about the Australian Business Number (ABN), a single identifier for all business dealings with the tax office and for dealings with other government departments and agencies.

http://ato.gov.au/businesses/pathway.asp?pc=001/003/021&mfp=001&mnu=59#001_003_021