I disagree with Mark Kleiman's assertion that there's something immoral about the principle of the Australian Prime Minister criticizing an American presidential candidate. As long as foreign leaders are just trying to use persuasion to influence voters but not use force or money, then they've got every right to do it and voters have the right to consider their arguments.
As a matter of policy instead of ethics, though, it's rarely a good idea. If the Australiam PM or his party stays in power through 2009, they may find he just created a significant problem for Australia that he could have easily avoided.
And as a matter of substance, what he said was complete nonsense, and will go nowhere in terms of persuading Americans. Someone else noted that the comment really had nothing to do with America, though, and is all about domestic politics. The Australian PM is sacrificing Australia's interest to help himself politically. Sounds familiar.