Have a look at the thread linked above and the other US threads in the Car hire section.
To clarify/summarise:
Almost all Australian credit cards only cover the 'excess' on the rental car waiver (as mentioned, it is not insurance). The exceptions are Amex Plat and Centurion Charge cards, which provide Loss Damage Waiver cover for the rental car only (they do not provide third party legal liability cover, nor does any Australian issued travel insurance policy provide this for rental cars, as far as I'm aware). Despite what is mentioned in the thread linked above, as Mwenenzi says, excess is rarely applicable in the US - you either have no insurance, or full cover (where CDW/LDW and LIS/ALI taken or included), with no excess.
Forget using credit card - there are plenty of other options.