As equus states below, with a couple of points of further clarification.
To further clarify, when insurance is purchased from car hire companies in the US, there is no excess.
With regard to Amex Platinum Charge Card, although it covers LDW/Deductible, it does not provide third party property damage, third party injury cover or legal liability cover. This means that you will still need to purchase this in the US - known as various names by different car hire companies, including Additional Liability Insurnace (ALI), Supplemental Liability Insurance (SLI) and others. Amex Plat Charge Card's included legal liability cover also specifically excludes cover when using a motorised vehicle, which would include a hire car, as do most, if not all, other credit card policies and bought travel insurance policies.
As discussed in numerous other threads on this forum, the best options for non-US residents, including Australians, when renting a car in the US are:
1. Hiring from the likes of Hertz, who often provide packaged rates including all insurances, often for less or only slightly more than the price for US residents excluding any insurance;
2. Booking through the UK based websites of the major car hire companies (e,.g. Avis, Dollar, Thrifty) which provide packaged rates including all insurances;
3. Booking through third party consolidators/aggregators/brokers, which also provide packaged rates including all insurances. Members on this forum recommend the likes of argus, driveaway holidays, netflights and others.
4. purchase a separate policy which specifically covers LDW and third party legal liability. There are a number of them available, usually from the UK, with cost starting at approx. AUD 100. Again, search this forum for further details.