Once your ship leaves port, even if you're cruising Australian waters, you’re no longer covered for hospital or medical expenses by Medicare or your private health care provider, meaning they won't cover any expenses incurred on-board the ship. You’re also not covered by Medicare and private health when you’re on land for cruise excursions.
However, you can purchase a Cruise Pack to cover you when you’re on a cruise. If you’re taking a cruise within Australian waters, you will need to select a domestic policy with the Cruise Pack add-on and select "Australia Cruise" as the destination/region. You can find out more about choosing your cruise region here.