Depending on where in the US and where in Australia, they’re roughly comparable in distance from Japan, so I’d think cost would be the issue.
If you read the original article, the problem is that it’s currently not possible to send french fries from the US west coast so McDonalds is resorting to air freighting them and sending them from the east coast. If Australia is the same distance away as the west coast, then the cost should be comparable and lower than the alternatives McDonalds is considering.
I think you might be underestimating just how many potatoes we’re talking about here. Importing from another country might be a valid mid to long-term solution, but I doubt there is, right now, enough potatoes in Australia – or anywhere else in the world – to meet the Japanese demand. Even if there was a massive pile of McD-spec’ed potatoes lying around somewhere, it’s doubtful local processing plants would have the time to prepare them on such short notice.
2014 really was annus horibilis for McDonalds Japan…