In the customers section you have a Business Identifier section. This Business Identifier section would be more useful to me in the Suppliers section. As my Customers don’t generally hold an ABN as they are individuals, where as my Suppliers all hold ABN’s.
Go to Settings → Custom Fields, under Supplier click on Custom Field. Call it ABN or Business identifier (or anything you like really). Hope this help.
You can set up a Custom Field to include the suppliers ABN. Go to Settings, select Custom Fields and click Supplier - New Custom Field.