Prime Minister John Key has described trade talks that started in Melbourne on Monday as marking an important milestone in New Zealand's free-trade agenda.
The eight-country Trans Pacific Partnership could free up access to crucial export markets in the world's largest economy.
Efforts to start the talks stalled after the election of Barack Obama as United States president last year, but he has now committed to the negotiations.
Other countries involved in the talks are Australia, Chile, Brunei, Singapore, Vietnam and Peru.
Mr Key says the talks are aimed at reaching a deal among eight countries spanning the Pacific.
He says an agreement, as well as opening up access to the US, could lead to a free-trade area for the entire Asia-Pacific region.
Mr Key says the second round of negotiations will be held in June.
New Zealand negotiators are expected to push for improved access for farm exports at this week's discussions.
There has been stiff opposition from the US National Milk Federation, which believes improving access for New Zealand's dairy products could cost its industry $20 billion.
The Dairy Companies Association of New Zealand has moved to reassure farming lobby groups in the US there will not be a flood of New Zealand product on the American market if a free-trade deal goes ahead.
Association executive director Simon Tucker says the US milk federation should be concentrating on export opportunities that the Trans Pacific Partnership will present in the Asia-Pacific region.
The Council of Trade Unions fears a deal could come at the cost of improved access to New Zealand for American financial firms.
It says this could lead to a proliferation of the types of financial products that caused the failure of some banks in the US.
Call to consider needs of workers
Trade unions from Pacific countries are calling for workers' rights to be given priority at the talks.
Council of Trade Unions president Helen Kelly says that, for the first time, her organisation has joined its American, Australian and Singaporean counterparts in signing a declaration to present to the governments involved.
It urges them to consider workers' needs and ensure that the negotiations are transparent.
Ms Kelly says some free-trade agreements favour trading rights over labour rights. She says the unions are trying to ensure that no country obtains a competitive edge by rejecting good labour standards.