Appealing a Centrelink decision first involves requesting an internal review of the disputed decision. This is a review that is done within Centrelink by Centrelink.
If a person is not satisfied with the outcome of that review, they can seek external review, that is, a review by an external agency. For Centrelink disputes, that agency is the Administrative Review Tribunal (ART).
A person cannot seek external review of a decision without first going through the internal review process.