Lordy, how hard is this? Laptop charger has a three pin plug. Japan uses two pins. Plug adaptors are available for Oz to Jap but not with any space for the earth plug. A multi country adaptor will work, but such a cost. Oz to Euro and Euro to Japan will work too, but how messy. It seems the adaptors are readily available in Japan and hotels lend you one for free. But I like to be prepared.
What do Australians who like to be prepared do? They hacksaw off the earth pin from their laptop charger or drill a hole in the standard Aus to Jap conversion plug to accommodate the earth prong. I think we will do the latter.