Japan has been allowed to enter the Trans Pacific Partnership trade talks by the 11 countries already in negotiations.
Trade Minister Tim Groser announced Japan's entry had been agreed on from Indonesia where TPP talks were held on the sidelines of the APEC trade ministers' meeting.
New Zealand has been cautious about Japan's entry into the talks because of its protectionist policies and high tariffs especially in horticulture and agriculture.
Mr Groser gave New Zealand's final approval after meeting with Japan's Minister of Economic Revitalisation, Akira Amari who had assured him Japan was committed to a comprehensive agreement.
"New Zealand was seeking assurances that Japan is committed to achieveing the ambition that has been set for TPP and could positively contribute to the momentum of the negotiations."
He said Japan was already one of New Zealand's largest trading partners and one of the region's economic heavyweights.
"Japan's participation in TPP would add substantial economic weight to the group. More significantly, it would reinforce the strategic vision we have for TPP, including the agreement's potential to serve as a platform for wider trade and economic integration across the Asia-Pacific."
After TPP talks in Honolulu in 2011, US President Barack Obama said he expected an agreement would be completed by the end of this year.
Mr Groser said the eleven other countries - Austalia, Brunei Darussalam, Canade, Chile, Malaysia, Mexico, New Zealand, Peru, Singapore, the United States of America and Vietnam - were assured by Japan's commitment to achieving a comprehensive and ambitious agreement as soon as possible.
Japan will formally join after those countries have made the required legal changes.