A Trans-Pacific Partnership (TPP) free trade agreement could boost New Zealand's economy by more than $3 billion a year, Prime Minister John Key says.

The 15th round of TPP talks opened in Auckland on Monday and Mr Key says the government will give the negotiations "a big push" because New Zealand has so much to gain.

"The Asia Pacific region will drive economic growth opportunities in coming years and we can't afford to sit on the sidelines," he said at his post-cabinet press conference.

"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 and Canada.

Mr Key says the government wants a TPP that would eliminate all tariffs.

"TPP participants already take 38 per cent of our exports," he said.

"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.

"Forecasts suggest the gains for New Zealand could be as high as 1.4 per cent of GDP, roughly $US2.9 billion ($NZ3.5b)."

Earlier on Monday free trade opponents complained they had been locked out of the Sky City Convention Centre where about 500 negotiators are gathered.

Campaigner Jane Kelsey says she and her supporters will only be allowed in on Friday, known as stakeholders day.

"We turned up this morning and found we were locked out of the entire convention centre for all 10 days except the so-called stakeholder day," she said.

Green Party co-leader Russel Norman says the restrictions "confirm suspicions that the negotiations are inherently anti-democratic".