I am only (slightly) familiar with the PhD program at UWa, but I believe that it is similar to other programs. At UWa you apply simultaneously to 2 programs: your ‘home’ subject and the Astrobiology program. So (with an astronomy UG degree) you would apply to the astronomy dept and the astrobiology program. If you are accepted by both there are a series of courses that you do and research work that you do. The ultimate degree would be “PhD in Astronomy and Astrobiology”. A Masters in Bio would not be necessary.
What will be important is research experience: do you have any? do you have a research internship lined up for this summer?