There are publicly available procedures and forms in Australia that enable you to create a binding Advance Care Directive. However, not every Australian state or territory requires you to do so.
If a specific form isn’t available (like in NSW where the “common law” applies) you can make your wishes known by writing them down yourself.
Signing and having the document witnessed is recommended, and it’s required in states and territories that have statutory Advance Care Directives.
Where forms are available they can generally be found via links on the websites of state and territory trustees, departments of health and seniors associations.
Because the law regarding Advance Care Directives varies Australia-wide, you should get legal advice to help you understand how to put together an Advance Care Directive, and what the possible consequences and outcomes are likely to be.