Is it illegal for a medical professional to disclose information of a young person concerning their sexuality or gender identity to the young person's parents or guardians?

A health agency must not disclose health information (which can include information regarding sexual orientation or gender identity) about a young person to their guardian provided to them in the course of the provision of health services, except where there is a specified exception such as where there is a threat to the life or health of the young person.  
 
Health professionals are expected to determine whether patients under 16 years are able to understand their condition and make decisions about their treatment, and if so, should only share confidential information with guardians where the patient has provided consent, or where there is a risk of harm.
 
Health agencies must designate a person (or persons) to deal with complaints regarding perceived breaches of privacy, and young people may also complain to the Privacy Commissioner.  Any disclosure of personal information must also be in accordance with the Privacy Act.