Applying for Medicare prior to emigrating to Australia?

If you are talking about Australia's Medicare program:
You aren’t eligible for Australia Medicare if you:
•are a permanent resident of Australia and have been absent from Australia for 12 months or more, don’t live in Australia or haven’t returned to Australia to reside permanently.

U.S. Medicare rarely covers any medical issues if you are outside of the United States.

https://www.medicare.gov/Pubs/pdf/11037.pdf