If you obtain a Green Card, you are generally not eligible to purchase federal U.S. Medicare until you have been a legal resident for at least 5 years.
The United States does not offer a dedicated retirement visa, meaning mature Australian adults must qualify through traditional immigration pathways. mature au usa
The U.S. does not have a reciprocal healthcare agreement with Australia. You will not be covered by your Australian Medicare while in the U.S.. If you obtain a Green Card, you are