US presidential candidate Hillary Clinton has just made clear her position on the Trans-Pacific Partnership mega trade deal with Australia, New Zealand and other key Pacific rim nations - she won't sign it.

"My message to every worker across America is this: I will stop any trade deal that kills jobs or holds down wages, including the Trans-Pacific Partnership," the Democratic contender said in her latest campaign speech.

"I oppose it now, I will oppose it after the election, and I will oppose it as president."

Her only opponent, Republican Donald Trump, fiercely opposes the TPP.

That means the only chance for the 12-nation agreement to succeed is for the US Congress to ratify it during the so-called "lame duck" period between the presidential election on November 8 and the inauguration of the new president on January 20.

That's considered to be a possibility, but having both presidential candidates firmly opposed to the agreement doesn't help.

President Barack Obama's position is that the US must ratify it, or China will set the global trade agenda.

To take effect, the agreement has to be ratified by at least six countries that account for 85 per cent of the group's economic output, which makes the US and Japan essential.

The TPP is one of the world's largest trade liberalisation deals, covering 40 per cent of global trade and 800 million people.

The partner countries are New Zealand, Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, Peru, Singapore, the United States and Viet Nam.