Not sure if you could get Japanese in Australia without first getting them established along the West Coast of North America. The route there is easy, just follow the ocean currents. Once the Japanese shogunate/imperial government is interested in what they can find in North America (gold), maybe they'll explore elsewhere and eventually find Australia. Perhaps they'll find the gold in Queensland? Probably the best way to get the Japanese in Australia, otherwise its even more outlandish than Japanese colonisation of North America.
I'm also not convinced you could get Japanese colonisation of ANYWHERE (at least pre-modern) with a POD later than the Mongol Invasions of Japan. Maybe a hyper-successful Ashikaga Shogunate could work, though.