To study at UWA for up to 12 months you should be an enrolled student at your home university or college at the time of application.
The requirements are:
You'll need to have the appropriate prerequisites for the units that you wish to study at UWA. Generally, first year courses can be taken with no background, while higher level courses can only be taken if you have the appropriate prerequisites or have taken similar courses.
Check the units you are interested in taking for individual prerequisites.
Make sure you meet the University's prerequisites as well.
If you are from a non-English speaking background you'll need to meet the University's official English language requirements.