He is not eligible for Japan because at the time of his first game for NZ, he did not hold Japanese citizenship.
http://en.m.wikipedia.org/wiki/FIFA_eligibility_rules
Unless geopolitical changes play a role, players are generally not allowed to switch nationalities if they have made senior appearances for one FIFA-recognised country in competitive fixtures. Friendly match appearances do not commit a player to one country (though a player is usually able to switch to another national side only if he held the nationality of the second country at the time he represented his first choice national team)
He will only ever be available for NZ until the day he dies regardless if he holds a passport or renounces his citizenship.
How is this not getting through to some people (not directed at you Ryan)