Its a AU government requirement for any income from "self employement'. Also Apple will not accept your application for a paid books account unless you complete all requirements in the Contracts and Banking, found in your iTunes Connect account,
If you search online for ABN, there is a website where you can apply. In fact any person in AU can apply and hold an ABN and ignore the tax aspects UNTIL you earn, - outside any normal employment, AU$1 after that you need to declare any income from Apple. The website will give you all the information you need.
Paid account process. Everybody/ You need to obtain a US IRS EIN number. Use the links in the post above to open and check the website. Download teh SS4 and porint it off. Fill it in where you can... the US tax agent will ask if you have the form and will go through it numerically with you. Anything you dont understand leave blank and tell teh agent when he/she asks about the box number.
Use the telephone number and timing advice I gave above. You get your number at the end of the phone call. Use to send in your application, to Apple and it will take 2/3 weeks for apple to confirm its valid.
Applicants in Australia are required to suppply an ABN number, if you are an employee, by having a paid book account you become a self employed eBook Publisher and any income is taxable.
You will need to check your status when you go to the ABN application website. I doubt its every month and suggest its incuded with your annual tax return.
The process is quite easy.. it just sounds full of complications. Deal with it one item at a time.
I suggest you open your iTunes Connect account and open teh Contracts and Banking section. Open and fill in the forms... make notes of what you cannot or do not have and sort those problems till you do. Just sign out of iTunes Connect and ignore the forms. When you have more info.. go back and add or correct the forms until they are ready to submit.