Introduction: People with intellectual disability (ID) experience barriers in accessing mental health care. Recommendations have been made to implement specialist intellectual disability mental health (IDMH) services in Australia. However, there is limited evidence to inform service development. Method: Family members and support persons of people with ID (n = 42) completed an on-line survey about support for, and operation of, a tertiary IDMH service in New South Wales, Australia. Results: Participants agreed that a tertiary IDMH service would assist in meeting the needs of people with ID. Key service features included that it be delivered within the public health system, by psychiatrists and psychologists, provide face-to-face clinical contact and advice. Key service areas included behaviors of concern, self-harm, assessments, and interventions. Conclusion: These findings suggest support for a tertiary IDMH service and how it could be delivered. Further research is required from the perspective of people with ID, mental health staff, and clinical experts.