Talks by trade ministers involved in the Trans Pacific Partnership have been pushed out by a further day - a sign the agreement is on the verge of a breakthrough.

Prime Minister John Key leaves New York today while Trade Minister Tim Groser remains in Atlanta at the trade talks. Mr Key said those were due to end early this morning but had been pushed out to Saturday (Sunday NZT).

Mr Key said the deal would be worth it, but has recently been talking up the benefits of it non-dairy sectors and warning that it was not as favourable for dairy as other sectors.

"We model the tariff reductions, the change in quota access and all of the other ancillary benefits of free trade agreements. And we know in the case of China, the reduction in tariffs over a period of time were a bit over $100 million. And the other benefits were valued at over $1 billion. TPP is larger than that, even with the current dairy deal."

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He said while the Government had a responsibility to do the best it could for the dairy industry, it could not ignore benefits to other industries.

"There will be many other sectors that will be, on the basis we sign TPP, ecstatic about what happens."

Media reports in the United States report that a deal is close and the US President Barack Obama had rung Australia's Prime Minister Malcolm Turnbull to discuss it. Dairy remains the main stumbling block from New Zealand's point of view.

Meanwhile, University of Auckland law professor Jane Kelsey said longer monopolies for big pharmaceutical firms were the main obstacles to signing the deal.

She said future New Zealand governments would have to stump up hundreds of millions of dollars more to Pharmac if the deal went ahead.

Prof Kelsey said health economists believed every added year of protection for biologics would cost New Zealand "many tens of millions of dollars in current spending, and much more in the future as more biologics come on stream."