Per the State Department’s travel advisory’s the U.S. Embassy in Australia has noted the following:
Australia has enacted a travel ban on all non-residents and non-Australian citizens entering Australia. Individuals may request exceptions to the entry restrictions at this link.
The Australian government requires all individuals traveling to or transiting through Australia, with limited exceptions, to provide evidence of a negative COVID-19 (PCR) test at time of check-in. The test must be taken 72 hours or less prior to departure. Additionally, masks are mandatory for passengers and crew on all international and domestic flights.
Further details are available on the Australian U.S. Embassy’s website.