Print on Demand books through CreateSpace or Smashwords withhold 30% tax on your royalties unless you have a US tax ID.
If you are an Australian, but do have a US tax ID you only have to pay 5%... so you end up with a whopping 25% more money from your royalties.
In those cases because we do not have US tax ID's we need to get one.
The process for getting a US Tax ID for an individual is a pain in the butt, however through trawling through a bazillion different US tax related forums and sites I was able to find a great way around it
You can get an EIN (Employer Identification Number also known as a Federal Tax Identification Number) in your business name, or if you do not have a registered business name, in your own name.
If you call the IRS on 267-941-1099 and ask for an EIN they can issue it over the phone. It takes about 10 minutes and you will need your Australian Tax File Number.
I recommend using a service like Skype to make the call so you can do it cheaply.
Once you have that you just fill out a W-8BEN form and send it to CreateSpace/Smashwords,etc
Instructions about form: http://www.irs.gov/pub/irs-pdf/iw8ben.pdf
The tax treaty between the US and Australia is Article 12
This allows you to pay only 5% withholding tax on book royalties
For the reason you are using the W-8BEN form you put "Australian author self publishing via US distributor"
You only need to get an EIN once