Be in the know before you go
A valid passport is required to travel and should have at least 6 months validity after your return home. You should have at least 2 blank pages per country that you are visiting. If your passport does not meet this requirement at the time of departure, you can be denied boarding for your international flight and may consequently incur additional expenses.
According to the information that we have, US citizens need a visa to enter Australia as a tourist with a US passport. However, we always recommend that you double-check this for last-minute changes with your local consulate.
Most U.S. passport holders traveling to Australia for tourism or business purposes for less than 90 days can obtain an Electronic Travel Authority (ETA) at the Australian Government Department of Immigration and Border Protection website: https://immi.homeaffairs.gov.au/visas/getting-a-visa/visa-listing/electronic-travel-authority-601
According to the CDC, Australia has no risk of Malaria. Nonetheless, you should be aware and consult your physician for any pre-travel recommendations.
There is no risk of yellow fever in Australia. The government of Australia requires proof of yellow fever vaccination only if you are arriving from a country with risk of yellow fever. This does not include the US.