When a doctor bulk bills you, you don’t pay anything.
You probably already know that bit!
The doctor then receives from Medicare a certain amount relative to the time and complexity of the consultation. And nothing more.
The amount that Medicare pays the doctor, the “rebate”, may or may not be sufficient for the doctor.
The doctor can choose to charge no more than the rebate or as much as they wish, in the same way that any other service provider can charge for their service.