Comment by OkayPhysicist

This gives the identity brokers full insight into what sites you are visiting. Worse yet, it consolidates the information in one, easily leaned-on place: If I provide an ID to 3 non-Australian companies, sure, those companies know who I am, but the government or other companies would need to extract that information from each one. With an OAuth scheme, all that information is in one convenient place for the surveillance freaks.