In order to do things like claim GST on expenses, you need to prove that your stallion interests are being conducted as an enterprise as opposed to a hobby.
You will need a comprehensive business plan and efficient record-keeping methods. You may also need to detail the professional advisors you rely upon when making business decisions such as selecting horses to purchase, organising matings etc.
Effectively, you need to prove to the Tax Office that there is a reasonable expectation of profit in the long term.
This further emphasises the importance of selecting commercially viable stallions, as it is difficult to prove that there is a reasonable expectation of profit with a stallion that you will struggle to book mares to.