He believed it was important to attend the Apec summit to keep momentum on trade going.
"It's obviously a pretty quick trip so we are in reality away a couple of days. If something significant happened, we would return. We'll keep an eye on things obviously, and if we really needed to return we would."
He said the difference art Apec was that the grouping of 21 economies had consistently been in favour of free trade and had a long standing goal of a free trade area across the Asia Pacific.
"Now for the first time ever really, we've got the United States under President-elect Trump going into a very different space."
He said he would urge the other leaders to maintain the agreement even if the US dropped out.
New Zealand does not currently have a free trade agreement with Japan and the TPP was a way to secure that as well.
"If we can't get there with the United States, Japan would still be worth it."
Trump will not be at the summit, which will be attended by US President Barack Obama.
Key said he was still hopeful Trump could be talked around, saying what was said on the campaign trail often differed from reality in government.
"I mean it wasn't that many weeks ago that Donald Trump didn't have terribly nice things to say about Mitt Romney and Mitt Romney didn't have very nice things to say about Donald Trump. If you believe the media, they're meeting this weekend.
So he hasn't had great things to say about TPP but maybe with some coercion and maybe with some changes we could agree to ... maybe it's possible to get him there."
Key does not have formal meetings with the super power leaders - Russia's President Vladimir Putin, China's President Xi Jinping or the US President Barack Obama, who is at his last Apec.
However, he did expect to get some time on the margins with them.
He will also be trying to get a word with Japan's PM Shinzo Abe. Abe was the first world leader to meet with Donald Trump in person and Key will want Abe's take on whether Trump can be budged on the Trans Pacific Partnership.
Key said he would also raise Japan's decision this week to send its whaling boats to the Southern Ocean, a step Foreign Minister Murray McCully has criticised.
Key expected to catch up with Australia's Prime Minister Malcolm Turnbull for a drink at some point.
Key will also meet fellow TPP leaders, Peru's President Pedro Pablo Kuczynskil and Chile's Michelle Bachelet, as well as Canadian Prime Minister Justin Trudeau, who is facing Trump's pledge to renegotiate the long standing NAFTA agreement between Mexico, Canada and the US as well as the TPP.
