I've been through the process and it was quite easy. You need to get an EIN from the IRS. You can get it over the phone, and it takes about ten minutes. I recommend filling in the paper version of the application first, so you have all the details ready when you have the conversation with the IRS. They issue the EIN on the spot.
Once you have the EIN, you can complete the application with Apple. Wait about seven days before fillling in the application. If you send it earlier, you will end up having the application bounce the first time. That's because Apple verify the EIN you supply with the IRS database, and it takes a few days for new EINs to appear in the database.
If you apply as a company, you will have to supply proof of ABN and GST registration. (Scans are fine.) Apple will send instructions for how to submit these.
Australia has a tax agreement with the US. Any tax withheld by Apple in the US will be credited against your tax here. You do not need to file a US tax return.
Overall, the process was quick and painless for me, no problem at all. Only glitch was that my spam filter ate messages from do_not_reply@apple.com, so I saw an email from Apple only a few days after it actually arrived. So, make sure mail from this address will actually be delivered to you.
You can get started here and here.
Michi.