I hear some countries require that you have at least 6 months before the expiry date on your passport from the date you enter (or possibly the date you intend to leave).
Is there a limit on passport expiry date for entry into Japan?
If it makes a difference, I have Australian and British passports.
Japan does not require the six months.
Any foreign visitor who wishes to enter Japan must have a passport, which will remain valid during the period of stay.
from http://www.jnto.go.jp/eng/arrange/essential/visa.html . There are numerous less-official surces stating it even more explicitly, and statements from people who asked an embassy directly.
No such requirement applies here. Your passport simply must be valid for the duration of your stay in Japan.
According to the UK Government for your British passport: "Your passport should be valid for the proposed duration of your stay. No additional period of validity beyond this is required."
According to the Consulate-General of Japan--Melbourne: "The passport must remain valid during the period of the stay but there is no minimum period of validity required."
According to timatic which the airlines use to check passport and visa validity, either your UK or Australia passport just needs to be valid on arrival.
