New Zealand will be among the bigger gainers from the Trans Pacific Partnership Agreement, boosting exports by around 10 per cent by 2030.

A study by the World Bank on the controversial trade agreement claims that while New Zealand will see a much smaller boost to economic output than the likes of Vietnam and Malaysia, the boost from being part of the TPPA would be far bigger than that accrued by Australia, Canada or the United States.

Calculating increases in trade and economic output, New Zealand ranks fourth out of the 12 members on both counts.

Across the trade agreement, the World Bank estimates the gains will be small in the short term but would increase over time.

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"The benefits are likely to materialise slowly but should accelerate towards the end of the projection period [of 2015-2030]," the report's authors estimate.

"The slow start results from the gradual implementation of the agreement and the lag required for benefits to materialise."

The study estimates that New Zealand's exports would be 10 per cent higher than it would otherwise be by 2030.

This is much smaller than the 30 per cent boost to Vietnam and 25 per cent boost to Japan, but outstrips the 5 per cent boost to the United States and even smaller gains to Australia, Canada and Mexico.

"The largest gains in GDP are expected in smaller, open member economies," the report claims, predicting gains from lower tariffs and non-tariff measures in large export markets, and from greater integration in regional supply chains.

In terms of boosting New Zealand's gross domestic product, the World Bank sees New Zealand's economic output 3 per cent higher than it would otherwise be by 2030.

This is a smaller gain than Vietnam (10 per cent) and Malaysia (8 per cent), but a bigger gain than Japan, Mexico, Canada, Australia and the United States.

The study estimates that countries left out of the TPPA agreement will suffer. Even though trade overall would be boosted, with Russia seeing a slight gain to its GDP from spillover benefits, the economies of China and India would be about 0.2 per cent smaller than they would otherwise be.

Thailand, hit by declining exports from being left out of the agreement, would see its economy about 0.8 per cent smaller than otherwise.

It is possible that membership of the trading block could be extended, with Taiwan already lobbying to be included if new countries are added.

This week TPPA critic Jane Kelsey accused the New Zealand Government of "obsessively secrecy throughout the TPPA process" with reports from Chile that New Zealand will host a signing ceremony on the agreement on February 4.

"[W]e have to get confirmation of what is happening in our own country from offshore," the Auckland University Professor said.