It is not mandatory to contact accredited supervisors prior to applying. If you're successful in your application, the University will allocate suitably qualified supervisors to you (refer to section 10 of the Higher Degree Research Regulations).
Should you contact an academic and they indicate they are willing to supervise you, this does not guarantee you will be admitted to the program or that they will be one of your supervisors. The availability of suitable supervisors is assessed as part of your application for admission (refer to section 10 of the Higher Degree Research Regulations).