New Zealand's Council of Trade Unions said removal of trade tariffs in the Pacific would take away an important source of funding for social services.
The council's secretary Sam Huggard presented evidence to New Zealand Parliament's Foreign Affairs, Defence and Trade select committee examining the progress on the PACER-Plus trade agreement.
Mr Huggard said taxes on goods from Australia and New Zealand supported health, education and social services, for which there were few alternative funding sources.
"One of those would be a consumer tax like a GST or a VAT and in the end those taxes hit the poor much stronger than they hit people who are well-off," said Mr Huggard.
"So, if a primary way of raising money is going to be one that negatively impacts on the poor then that's not something we could ever support."