I am Australian Developer, and have done all the tax stuff. You need to
- Register for an ABN Number for Australia (free if you do it as a sole trader)
- Register for GST (again free)
- Call the US and get an EIN (free except for the phone call)
Once you have all that you can fill out the last section of the tax info stuff in iTunes connect. In the tips PDF that apple supply with the form there is the number for external people dialling the US. Simply add 0011 1 to the front of it. You can only call during US business hours. (I'd find it for you now, but my internet has been shaped for the next few days, so it would take an eternity to download)
When you've done that you submit the form. Shortly afterwards apple will email you a filled out copy, that you need to sign and mail back to them.
I should note that if you want to charge for your apps, and don't want to be taxed at 30% (on top of the 30% that Apple already takes), then yes, this is Mandatory. Also you cannot sell to Australia unless you are registered with an ABN and for GST. If you just want to give your stuff away then no, you don't need to do any of this.
Finally you need to submit a copy of your GST and ABN Number registration through the main page of iTunes connect, down the bottom.