Bill English says the uncertainty around US President Donald Trump is helping keep the Trans-Pacific Partnership (TPP) alive.
One of the first moves Mr Trump made as President was to tear up the agreement, which New Zealand had already ratified. But with Mr Trump's approval ratings in freefall and growing speculation he could be removed from office or forced to resign, Mr English is holding the door open for the US to return.
"I think it's one of the incentives for the other 11 countries to keep going," the Prime Minister told The Nation on Saturday.
"The US could come back to TPP if there's a TPP operating, and if it remains basically unchanged from the one that the US negotiated."
Mr Trump has flip-flopped on a number of issues since becoming President, and if he has a change of heart on the TPP, Mr English won't hold his prior stance against him.
"Well, whether Donald Trump's there or not, they would always be welcome back because that's the big prize."
Trade Ministers from various countries are meeting in Vietnam on the fringes of APEC, and New Zealand's - Todd McClay - will be co-chairing the meeting.
Twelve countries signed up to the TPP, making up about 40 percent of the world's economic output. All 12 nations needed to ratify it to make it law, but the United States' withdrawal has put it in jeopardy.
"This TPP meeting will be important for the future of the agreement," said Mr McClay.
"This is a chance for the 11 remaining countries to collectively decide upon a process for the next steps."
Mr English, who has recently been in Japan, says New Zealand and Japan are now effectively leading the next phase of the TPP negotiations.
He admits however depending on how the TPP agreement changes without the US, it may need to go through Parliament again.
"We haven't had legal advice yet on exactly… We'll get the advice about what's required, but we don't imagine that a successful TPP will require significant change in legislation."
And he expects Labour to back it.
"They should. Whether they will or not is another matter, because just because something is a good idea often means the Labour Party don't vote for it."