Your first step would be to look here - Future students
The minimum requirements for undertaking a PhD as well as information on the application process can be found here - Standards and admissions
Are you aware of the fees for international students? As of 2016, they are about $40,000 Australian per year
In order to be accepted into a PhD program, The University of Melbourne require a First Class equivalent grade point average (i.e., overall average for the past two years of study equal to or above 80%).
Because of the requirements for highly effective communication in projects run within my lab, I require potential students to achieve an average IELTS score of 8.0 (written 8.5 with no band less than 8.0).