Prime Minister John Key says he sees a good chance of wrapping up a Pacific trade deal this week and warned political headwinds may scupper the pact if negotiations drag on too long.

Trade ministers from the 12 countries negotiating the Trans-Pacific Partnership gather in Atlanta later on Wednesday in a bid to close the pact, which would cut trade barriers and set common standards for 40 per cent of the world economy and be the biggest free-trade deal in a generation.

Speaking at the Asia Society in New York, Key said New Zealand was still not happy with dairy market access but agreement was closer on other tricky issues, including intellectual property.

CHRIS WATTIE / REUTERS Canadian dairy farmers walk with their cows during a protest against the Trans-Pacific Partnership trade agreement in front of Parliament Hill in Ottawa on Tuesday.

"I think there's actually a chance (the TPP) could get completed this week," he said. US elections are looming in 2016, which will make it more difficult to get the trade deal — which is opposed by many Democrats — through Congress.

"The window of opportunity to complete TPP is closing so you wouldn't say it's impossible to complete the deal if it doesn't take place in Atlanta, but it does become more difficult," Key said.

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TPP leaders will meet at an Asia-Pacific Economic Cooperation summit in the Philippines in mid-November, which Key said was "probably the last opportunity for 2015" to close the trade deal, already been more than five years in the making.

New Zealand's state-subsidised scheme for pharmaceuticals would be hard hit if the TPP mandates the US standard of 12 years data protection for biologic drugs, preventing generic alternatives from coming to market.

Malaysia's Economic Planning Minister, Abdul Wahid Omar, told Reuters on Tuesday the country was still concerned about rules on state-owned enterprises and government procurement and allowing foreign companies to sue host governments over regulations that damage their investments.

"These are all the issues that are being ironed out at this moment," he said on the sidelines of United Nations meetings.

"Free trade agreements are never easy  and when it comes to TPP, it's bigger than your traditional free trade agreement so therefore the issues are complex. I think we are moving very much closer and there is that hope among the various countries to see whether the negotiations can conclude in Atlanta."