Reddit reviews Guns, Germs, and Steel: Library Edition

Read this book which has a map very similar to this one and which provides an answer to your question. From what I recall it was something along the lines of JudahMaccabee's answer: The indigenous Australian / Papua New Guinean population could not be so easily replaced as, say, indigenous phillipinos because they were more adapted to the very peculiar environment than the Austronesians, whose food production package was not that suitable for the Australian mainland / New Guinean highlands.