If you have registered for ABN, it doesn't mean you register for GST. If you are subcontractor and you haven't registered for GST, employer cannot claim any GST on your subcontractor fees. All in all, I don't see any difference between ABN and TFN. TFN is applied when you work as a employee and employer pays tax on your behalf (deduct tax from your gross wages). ABN is applied when you work as a subcontractor, you pay tax yourself either quarterly or at the end of the year.