While I disagree with Rudd’s politics (obviously) what he is doing is little different from John Howard stating that Obama would be a problem as US President, or as President Bush speaking out against a Latham Labor government. The precedents have been set on both sides.
I’ve never seen anything quite so blatant, though...
I think it’s a bit different. The previous leaders only offered their assesment on what might happen if certain leader on the other country was elected. Here, Rudd was offering to coordinate their efforts—my understanding is that he would be willing to lend his advisors, etc., to get Hillary elected. Wasn’t there any law that is against foreign involvement in political campaign? Although IIRC, the SCOTUS allowed an international student to be a volunteer.