The RBNZ have been explicit on what they need to see before intervening in the currency market
It won't be entirely applicable to the Bank of Japan, for one thing the Reserve Bank of New Zealand is managing relatively (to the BOJ) small FX reserves and don't have as many bullets.
But nevertheless, it might be helpful (you're welcome):
Before intervening the Bank will need to be satisfied that all of the following criteria are met: the exchange rate must be exceptionally high or lowthe exchange rate must be unjustified by economic fundamentalsintervention must be consistent with the PTA;and conditions in markets must be opportune and allow intervention a reasonable chance of success.
(Note: PTA is the Policy Targets Agreement the RBNZ has with the government. It is different to PITA, which is what it is when evil speculators are meddling in your currency :-D ))
I reckon bullet point 1 hasn't been achieved, it fails at the first hurdle for the BOJ here.