While the question of why someone didn't do something is invariably difficult to answer, I think these two answers, the former by /u/AnAngryPacifist and the latter by myself, cover two major dimensions of your question already:
As the former answer notes, off-continent polities far closer to Australia had little interest and less luck trying to establish settlements in Australia, so why should 'China', however defined, have been able to do better, without secure control of the ports along the water routes required to reach it? Secondly, colonisation is incentivised by economic potential, strategic value, or ideological impulse, and Australia would not have offered that for any would-be Chinese coloniser. If even Taiwan, 150km off the Chinese coastline, held no particular interest until diplomatic circumstances caused it to gain prominence in the late 16th century, then why Australia, some 4000km away?