Additional drivers are recorded at the time of pick up as suppliers do not add these details to the reservation. Additional drivers need to present their driver's licence to the rental staff to be recorded on the Rental Agreement. The charge details and requirements are in the Terms and Conditions of rental under “not included in this quote”.
Keep in mind also that extra drivers have the option to be added before or after the pick-up date if they will not be in attendance upon collection of the vehicle by visiting a nearby depot to present their driver's licence as they will need to be added to the rental agreement before they can drive the hire car.
Read more about Adding Another Driver to Your Car Hire in Australia.