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?
Unfortunatley, no. They have already made a number of behind the scenes efforts solely on behalf of Beijing to undermine Taiwan's relations with Pacific Island nation with whom Taiwan had diplomatic relations.