Sheehy is leaving the company after five years to pursue other opportunities but will remain with the company until the end of July to facilitate a handover.
Ristevski will assume the role on 21 May and will bring more than 15 years of experience in senior finance and corporate roles throughout Europe, Asia and the United States to the business.
A search for a new company secretary is underway, Sheehy will remain secretary until 31 July.
“Michael has been with the company for the past five years and overseen a period of strong and consistent growth. I would like to thank Michael for his high level of commitment and service and wish him well for the future,” managing director John Guscic said.