MLS Mentor Program - Tips for addressing your mentor
This web site below provides some useful tips. When emailing your mentor (at least initially), Dear Ms/Mr (surname) is appropriate, with the exception of Judges, politicans and other notables. When in doubt, check a reputable source such as the one below. If your mentor has honours after his/her name, it is not necessary to refer to these when emailing, though essential for more formal forms of correspondence. As a general rule, always better to be too formal than informal - unless your mentor directs you otherwise.
Barristers - Dear Mr/Ms
County Court Judge - Dear Judge (include surname if preferred)
Federal/Supreme Court Judges - Dear Justice (include surname)
If in doubt, I'd be happy to assist h.green@unimelb.edu.au
http://www.dpc.vic.gov.au/index.php/resources/protocol/forms-of-address