if you did your nursing course in a cetified university (most Aussie ones are!), and pass the japanese national nursing exam, you are able to practice as a RN in Japan.
From looking at past exams, it's usually multi-choice, so even if you're a bit dodgey in kanji-writing, i think it's quite acheivable.
here's info about eligibility of foreign-trained nurses to sit the exam http://www.mhlw.go.jp/general/sikaku/5a.html (it basically says that you need 3+ years and 3000hrs+ worth of experience. And have graduated from high school and a certified university)