I must admit Don that the individual websites for those rental companies do not easily give up any info about cars with inbuit sat nav, I'd say it may be a matter of sending them an email to discuss it with them directly.
(The info I pasted previously came from a third party site, and while it had some info, ie Thrifty have some Holden Commodore's with GPS Nav, I think you had best confirm things with the actual rental company(ies)
Here's the contact details for Thrifty Car Rental:
Reservations
For assistance in making, changing or cancelling your booking please contact our National Contact Centre:
Reservations telephone:1300 367 227 (Australia only)
+ 61 2 8337 2790
Email:reservations@thrifty.com.au