The NGO, Oxfam, says the costs Tonga faces in joining the World Trade Organisation are huge with few apparent benefits.
It says the terms under which it has acceded to the WTO during the trade negotiations in Hong Kong may be the harshest demanded of any country.
Barry Coates, the executive director of Oxfam New Zealand, says Tonga has come under huge pressure from the rich countries.
He says Oxfam is concerned that Tonga has not had enough independent advice.
Mr Coates also says it has been a secretive process with a recently leaked document all that Tongan civil society had heard of the accession.
"There was a great deal of concern, from churches, from NGOs, from small business, about the deal that Tonga was about to sign up to. We were actually calling for Tonga to delay until they had a chance to do more research, more assessment, to get a better deal, and to consult within their society."