This was my first thought as well. Zheng He did not reach Australia, based on what I know of the Chinese sources, but that doesn't mean that is not the source o

This was my first thought as well. Zheng He did not reach Australia, based on what I know of the Chinese sources, but that doesn't mean that is not the source of the coins, as he famously paid for tribute. He was definitely in what is now Indonesia. Northern Australian aboriginal tribes have traded with Indonesia for centuries so a find of this nature shouldn't really be that surprising although it is definitely interesting.