The NGO, Oxfam, says Tonga is joining the World Trade Organisation on possibly the worst terms ever offered to any country.
Tonga is completing its accession during the current WTO negotiations in Hong Kong but Oxfam says the package the country has been forced to sign is appalling.
Oxfam New Zealand's executive director, Barry Coates, says Tonga will be forced to slash the tariffs on which it depends to pay for vital public services such as health and education.
He says it is very unfair that Tonga should be put under the sort of pressure where it has to slash tariffs to 15 and 20 percent.
"The terms of Tonga's entry are extremely unfair and it's not only the ceiling on tariffs - there are issues of the number of service sectors where they are having to make commitments and they are having to make commitments on property rights; they are having to dismantle regulations to promote the kinds of investment they want."