Getting into a US medical school as an international student is extremely difficult. Many of the private schools will have a few students each year who are international, but its not a lot.
It gets even smaller when considering MD/PhD combined programs. Please take this into consideration. If you are intending to go back to Australia to practice medicine, then this should be taken into account. The medical education system in US is post-graduate (after bachelors) whereas in Australia you attend medical school right after high school.