I'm about your size but a bit lighter... I rode a 153 Gnu Carbon Credit in Hokkaido last Dec and I found that it was insufficient for the conditions because we were lucky enough to get mad pow at the time. The whole time I was wishing I had a better board for the conditions so I bought one (Nidecker Megalight 158) before I went back last Feb. Looking at all the guys ripping on their Burton Fish(es) and Gentemsticks filled me with envy.
So if you decide to get one do-it-all board for use in Japan and Oz... consider sizing up a few cm. I think I would have been "ok" (still not ideal) if I had the next size up on my Gnu (156). Otherwise, I'd do what an earlier poster said and get a pow board for Japan and upgrade your Oz board when you get a chance.
The resident Japan specialists will probably chime in at some point with their advice. There are actually a few threads floating around discussing this point which can be found by searching "Niseko".
Last edited by ju87; 10-05-2012 at 05:42 AM.