In 2021-2022, China was the number two source of immigrants to Australia, after India. If Australia is more successful at attracting Chinese immigrants than Japan, the problem isn't availability of Chinese people wishing to immigrate, the problem is Japan's attractiveness as an immigration destination for Chinese people–and surely that's something Japan can improve if they try.

Also, there a lot of ethnic Chinese people in Australia who are actually from Malaysia or Singapore; Malaysian Chinese want to escape the discrimination of Malaysia's Bumiputera policy; many Singaporeans want an escape from Singapore's cramped living conditions (small apartments). So ethnic Chinese immigrants can be sources from places other than China proper.