Prime Minister John Key says the addition of the United States and Russia will change the dynamics of the East Asia summit.
He returned from Vietnam last night after attending this year's summit and meeting world leaders.
US Secretary of State Hillary Clinton and Russian Foreign Minister Sergey Lavrov attended as observers and confirmed they would join next year, taking the number of countries involved to 18.
Several members already have free-trade deals with each other and the summit has been working towards a comprehensive agreement.
Mr Key says that if the United States and Russia want to be part of that, they will have to join on the terms of the existing members.
The current membership is: the 10 members of the Association of South-East Asian Nations plus China, India, Japan, Korea, Australia and New Zealand.