[Robert Hay]

Hi Fiona

Under the Corporations Act 2001 Section 153 a companies ACN or ABN should be printed on all its public documents.

At Ku-ring-gai our abn is included on letterhead for all outgoing correspondence.

http://asic.gov.au/for-business/starting-a-company/how-to-start-a-company/australian-company-numbers/

Robert