From Monkey Island wiki
Australia is a far-off island/continent located outside the Carribean from which Ozzie Mandrill comes from and, while it never makes an appearance in the series, it serves as a pivotal aspect towards the series' overarching plot and mythology. Governor Horatio Torquemeda Marley travelled there to get away from his gubernatorial duties and to compete in the America's Cup. It's there that he meets Ozzie and teaches him the many secrets of the Caribbean, in particular the devastating power of the Ultimate Insult, essentially making Australia a critical location in the series. Additionally, there appears to be a whirlpool, apparently fueled by voodoo energies, just off the coast of Australia that magically transports anyone who falls into it to the shores of Monkey Island, which occurred to Haratio after Ozzie intentionally bumped his ship into the whirlpool in an attempt to end his life in order to secure the dark, voodoo secrets and lucrative prospects of the Caribbean that were told to him the previous night.
- Escape from Monkey Island (first mentioned)