You do not need to disclose your HIV status to your employer. Additionally, there is no requirement that your doctor disclose your HIV status on your medical certificate for sick leave. You can ask your doctor not to specify your HIV status on your medical certificate. Note as well that you have no privacy rights in relation to information provided to your employer.
Source: HIV/AIDS Legal Centre, Sydney http://halc.org.au/publications/guides-to-hiv-and-the-law/