Challenges if US doesn't pass TPP: Hockey

If the US doesn't pass a historic Pacific trade pact it would be the first time the country has rejected such a deal, Australia's Ambassador Joe Hockey warns.

Both presidential hopefuls Donald Trump and Hillary Clinton are opposed to the 12-nation Trans-Pacific Partnership.

"If America does not pass the TPP it will be the first time that America has ever rejected a free trade agreement and if America walks away from its own values ... then we've got much bigger challenges over the years ahead," Mr Hockey told Sky News from the Republican national convention.

The former Australian treasurer said hope now rests on the so-called "lame duck session" when Congress meets after the election but before the new president is sworn in.

"That is the last window for us to try and get the TPP through," he said.

"If it doesn't go through then, then it's unlikely to go through in the next year or two and that completely changes the nature of the agreement but we'll wait and see."

He described the deal as "hugely important" for the region and America.