, China is taking steps to get its currency recognised gloablly (become part of the IMF) so devaluing it and creating a broader trading range is a small step towards that. Going fully free and open in one hit could be catastrophic to China and ultimately the world so a few early measures is a wise approach.
Also while growth was slowing its a measure to make its exports more competitive hence the word manipulation used fairly often. On the surface it certainly doesn't look fair.
The initial devaluation probably hasn't had much of an effect on the NZ economy as our dollar has been weakening, however if this becomes a consistent tool used it certainly will have some effects. You are right there are no agreements internationally on setting or moving currencies but if countries keep doing this they won't be taken seriously in international trade and finance.
China is not part of TPP.....yet! Getting the current countries across the line including USA & Japan was no easy feat so having China in the mix would of delayed it by a few more years. China are in RCEP as are NZ but I am sure they are watching TPP developments with interest. They are also creating initiatives such as the Silk Roads and Asia Infrastructure Bank.