Sounds like Hertz are taking their teir structure in the US and introducing it to OZ, here are the US Benefits and qual levels:
In addition to benefiting from Hertz' expedited rental and return process,
#1 Club Gold members can earn higher program status with increased rental activity, ultimately becoming a #1 Club Gold Five Star or #1 Club Gold President's Circle member. Upon completion of 10 or more rentals within a 12-month period, customers are eligible for the Five Star level. At this level, customers receive with each rental, a complimentary one-car class upgrade based upon availability, coupon offers with greatly reduced blackout dates and a Free Rental Day Certificates after every 20 rentals.
At the end of each calendar year, members' rental activity will be
evaluated and, with a higher level of rentals, Five Star members will qualify for #1 Club Gold President's Circle.
#1 Club Gold President's Circle is for Hertz' most loyal and frequent
customers. Customers are eligible to attain President's Circle status after
completing 40 or more rentals within a 12-month period. This level provides guaranteed availability of a vehicle, with a two-hour minimum advance reservation, a guaranteed upgrade, (midsize and above reserved) and a vehicle parked in a specially designated President's Circle parking place that is closest to the vehicle pick up location in the #1 Club Gold area in major airports.
That was from a US press release and it may be out of date. Like the guaranteed car benefit!