As it usually happens in the case of United States of America, settling in Australia is quite a tough task. Turning into an Australian national simply means a promise to Australia and its citizens. The process of getting Australian citizenship for an American is no different from other citizenship processes.
An American individual by default qualifies for Australian citizenship if he is the son or daughter of an Australian citizen. For example if descendent of an Australian citizen is living in United States of America he automatically gets the citizenship of Australia. On the other hand, an American person can marry an Australian citizen to get the Australian citizenship.
Not only has this, to get Australian citizenship, an American also have an option of applying to migrate to Australia. This is clarified and settled on a points merit scheme. If at all possible the American person must have the necessary skill sets which Australia is in dire requirement of. Some professions and skill sets which are demanded quite frequently get favoured treatment. An individual would require to surf the internet to find out what are the existing professions which are demanded frequently for which there is a scarcity of expert workers.
United States of America permit its natives to have dual citizenship. An American lose his US citizenship when taking on another citizenship only if he do so with the clear purpose to give up U.S. citizenship - the objective of which can be revealed by the his official declaration or conduct.
It is also worth noting that even though the UNITED STATES OF AMERICA does not support dual citizenship, it does permits and respects it and that Australia has no trouble at all with the idea and has quite a number of dual citizens having citizenship of US and Australia.
A person shall be having both the citizenship once he or she qualifies and applies for Australian citizenship which he can anyway do after living in Australia for 4 years after their Australian spouse visa is issued. At the same time he must also declare that he does not want to give up US citizenship.