Most accepted credit card in Australia (particularly Melbourne and Sydney)?

Which is the one to use when I land? ATMs are plentiful, correct? Or bring USD to exchange? Help... thx David (NJ)

All will be accepted. For fee-free ATM withdrawal, your best bet is probably Citi, though they'll still charge a conversion fee iirc. All international airports and major cities have a Citi atm.

Visa and Mastercard are generally accepted almost everywhere (if you see an 'EFTPOS' sign, this generally means you can pay by credit card too).

American Express is accepted in the expected places and in most of the chain stores, but can vary at things like restaurants and smaller stores.

If you have a US Discover card, you can use it anywhere you see a Diners Club logo (or that accepts Diners Club), thanks to a partnership between the two networks in Australia. Don't ask if a business accepts Discover (as some may not know what it is): just ask if they take Diners Club. Diners is the least-accepted card in Australia, though, so you'll find it accepted more broadly at larger businesses and rarely at smaller stores.

Some businesses may also charge a credit card surcharge (most hotels do too), which is commonly in the field of 1-2% for Visa and Mastercard and 2-3.5% for AMEX and Diners/Discover. Many places have no surcharge,  but just warning you in advance as it's not something I've seen in the USA. :)

You will find that nearly everywhere accepts both Visa and MasterCard, surcharge free. AMEX and Diners are accepted in most major stores and in tourist locations, however usually attract a 1.5-2.5% surcharge. Most ATMs will charge a exchange fee and ATM fee. It is worth checking if your US bank has an Australian Fee Free ATM partner, for example Bank of America customers can use Westpac or St George ATMs Fee Free.

Marriott are One Major abuser of the system. The present MasterCard charge for Larger Hotel chains is Point . 8%, But Marriott charge 2% giving them an unholy Mark-up of 1.5%  on something that the Rest of The World usually does not even charge for. Governments should not allow this type of Rip Off to occur.