Unfortunately your're right, AMEX does not provide coverage rental car coverage in Oz, but some M/C & Visa cards do.

See an old thread here: http://www.flyertalk.com/forum/ameri...australia.html

Note that CC insurance is secondary & will only step in after your primary US insurance has been brought into the loop so if you do have a claim it may raise your future premiums at home.

I travel to Australia very frequently (US citizen, AU permanent resident) and have not had the same experience as travelmad478 where a credit hold was placed on my rental, although I have signed off to acknowledge the amount of the excess/deductible. Also worth noting is that with most Australian rentals you are responsible for payment of road hazards like flat tires, etc.