Do I need to provide a Tax File Number (TFN)/Australian Business Number (ABN) when buying shares?
Providing an Australian Business Number (ABN) or Tax file number (TFN) is not compulsory, but CCA is obliged to deduct tax at the top marginal rate plus Medicare levy from dividends, to the extent that they are not fully franked, payable to Australian resident investors who have not supplied their TFN/ABN or claimed a redemption. If you have an ABN and are making this investment in the course of a business or enterprise carried on by you, you may quote your ABN instead of your TFN.