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At a speech to Marlborough winegrowers in Blenheim yesterday, he jumped on Opposition promises to restrict land and house sales to foreigners as evidence that National was the only party which remained "open to the world".
He said free trade deals were the path to lifting incomes for New Zealanders, and reiterated National's commitment to a high-quality Trans Pacific Partnership Agreement (TPPA).
Mr Key described the 12-country TPPA as "the 400-pound gorilla" because it included economic giants Japan and the USA.
Labour has previously said New Zealand will benefit from the TPPA, but has not taken a formal position and faces resistance from unions.
Mr Key said: "Labour can't form a government without being with the Greens or [Internet Mana]. And we know the Greens are totally opposed to TPP.
"I think it is a fair representation that only a National Government will deliver a free trade agreement with the United States."
Labour's trade spokesman Phil Goff said Mr Key's claims were "a nonsense".
He said Labour had a strong track record, having signed off the China FTA, and would push ahead with the TPPA "if it preserved our bottom lines" and guaranteed "real progress on market access".
If a Labour-led Government helped secure a high-quality TPPA, he said National would be foolish not to support it from the cross-benches when it was ratified.
Some opponents have criticised the secrecy of the TPP talks, and are unsettled by its provision for companies to take direct legal action against governments through closed-door courts.
Mr Key said the debate about the agreement was clouded by misinformation.
He said every trade agreement was negotiated behind closed doors and then ratified through Parliament.
"If we go out and say 'Here's New Zealand's bottom lines', then obviously everybody ... knows where they can push us to.
"More often we achieve a lot better than we might have been prepared to accept."
New Zealand is also seeking a free trade agreement with Korea and with Gulf states. Mr Key said local meat producers alone paid $90 million in tariffs to get their products into Korea.
As a signal of its National's "openness to the world", Mr Key announced a commitment to put $10 million over five years into helping more New Zealand high school students learn the languages of this country's Asian trading partners - Mandarin, Korean and Japanese.
"Our next generation need to be able to work in different cultural environments as our international and trading links grow, particularly with Asian countries," he said.
Mr Key released the party's trade and foreign affairs policy at the Winegrowers Conference, at the heart of which is a target of increasing the ratio of exports to GDP from 30 per cent to 40 per cent by 2025.