I know some countries you can't lose your citizenship. I know if you give up you Australian citizenship you can just get it back an be a dual citizen that way. Not 100% sure about nz though. Imagine it will be the same and as he has already played for nz it shouldn't be a problem.
I know for sports japan often makes allowances, a lot of rugby players here have dual citizenship
P.s. gamba gamba, Osaka gamba. Top of j2 ATM