No, you are not obliged to tell an employer or prospective employer that you are HIV positive unless you work in one of the jobs outlined in the exceptions section below.
A prospective employer can only lawfully discriminate against employing you based on your HIV status (or the side effects of its treatment) if you are unable to perform the basic requirements of the job. There are very few jobs where this is likely to apply, but there are some jobs where you must disclose your HIV status, even if you believe that it does not affect your ability to do the job (see exceptions below).
Similarly, there are very few jobs where an employer or prospective employer can legally ask about your HIV status or require you to have an HIV test (see exceptions below). Employers are often unprepared for an employee’s disclosure of their HIV positive status and frequently over react because they think that the risk of transmission to others is much greater than it actually is. They may be concerned over situations that pose no risk (‘What if someone else drinks from your mug?’) or where there may be a slight risk that can be minimised by the use of standard precautions (‘What if you cut yourself and bleed?’).
If an employer dismisses you because you have HIV, or prevents you from undertaking certain tasks that would normally be part of the job, then this may amount to unlawful discrimination. Get legal advice as to your rights. Act quickly, because some legislation requires you to lodge complaints within 21 days.
Source: HIV/AIDS Legal Centre, Sydney http://halc.org.au/publications/guides-to-hiv-and-the-law/