Go to the Australian Tax Office website, and sign up for a Tax File Number as a temporary visitor. This will take you to an online form where you will be asked questions regarding your:
It takes 28 days from your application for your Tax File Number to be sent to you via post. Once you receive your letter containing your TFN, keep it in a secure place to prevent identity theft
In the meantime, your data will be sent to the Department of Home Affairs to check the validity of your visa. Your information is treated carefully according to Australia's privacy laws.