I can see them making a move like this, given their goal to be the dominant power in the region. They've got to see the western-allied Australia as an unwelcome neighbor.
But... given the fact that the current government there is one of America's closest friends, isn't it premature to call them a vassal state of PRC?
Yep, it is too late for Australia. Did you see how their government tried to look the other way when all this defector talk came out. They did not want to risk any tension with China (since they have substantial trade with Beijing), and thus would not even offer these defectors real asylum.