A few things about author tax for Australians.
1. Amazon deduct tax out of your royalty amount, at the time of payment. 30% if you haven't submitted your Australian tax details, 5% if you have. The 5% is deducted at payment time and is gone.
2. All you can do is claim tax already paid at tax time, using the yearly US$ rate for conversion, published on the ATO website. If your actual tax bill is less than that, you lose the excess. They dont do refunds of US tax. But if you have the income, the US tax decreases your Australian tax.
3. Any income from being an Author is taxed in Australia differently. If you apply to be classified as 24Z Special Professional Income, a completely different method of calculating tax is used, which should lower how much tax you pay, but it's calculated over a 5 year period. Filling the tax out on the online site is a real b*tch because you have to put 2 different sets of numbers in 2 places, but it can be done online. But first you have to ring the ATO and find out how to write to them to apply for it. It only applies on your author income, but it can keep you from bracket creeping. I did this and received a very healthy refund on the previous 2 years tax.
If you are earning well on book sales, you should be in 24Z. Its a process. It took 2 attempts a sending them the letter, and several hours on the phone to the ATO (they rang me), but it was worth it.
You do not need to be registered for GST or have an ABN. GST applies only if you sell from your own website, or from events. And it cuts in somewhere around the $80k mark, so very few authors will need to worry about GST registration.