Free trade opponents are preparing for another day of demonstrations in Auckland as delegates from 11 countries continue negotiating the Trans-Pacific Partnership (TPP).
They say the deal threatens New Zealand's right to make its own laws and an international doctors organisation has thrown its weight behind concerns that access to cheap drugs will be compromised.
Medecins Sans Frontieres (MSF) says leaked drafts of the US negotiating position show it is demanding aggressive intellectual property provisions that would roll back public health safeguards in favour of enhanced patent and data protection.
That would make it harder to gain access to generic drugs, MSF says.
The Green Party has voiced the same fears, saying the government's drug buying agency Pharmac won't be able to buy medicines at affordable prices.
The government says the public health service isn't up for negotiation and Pharmac won't be affected, but ministers acknowledge there are "sensitive issues" that still have to be addressed.
Prime Minister John Key is firmly behind the TPP, saying on Monday it could be add more than $3 billion a year to New Zealand's economy.
"The Asia Pacific region will drive economic growth opportunities in coming years and we can't afford to sit on the sidelines," he said.
"The aim is to wrap up a deal in the coming year - it's a challenging timeline but Auckland is our opportunity to build momentum."
The negotiations began in 2007, aiming to expand the existing free trade agreement between Chile, New Zealand, Singapore and Brunei to include the United States, Australia, Vietnam, Peru, Mexico, Malaysia and Canada.
"Collectively, the TPP economies represent $US21 trillion ($NZ25.6 trillion) in GDP - a comprehensive, 21st century agreement would provide many opportunities for New Zealand businesses," Mr Key said.