Malcolm Turnbull is urging the US Congress to pass the major Trans-Pacific Partnership Trade deal in the window before President Obama leaves the White House.
The 12-nation pact is in doubt with both US presidential frontrunners Hillary Clinton and Donald Trump opposed to it.
Speaking in New York on Sunday ahead of the annual United Nations General Assembly, the prime minister said he regrets the position of Ms Clinton and Mr Trump.
"There is a fair degree, I'd say only a fair degree of optimism that it will be ratified during the lame duck period," he told reporters, referring to the time between the November election and January.
"It will be a lot harder to do afterwards," he said.
"This is a window of opportunity and I believe it will be in America's long term interests as well the rest of the Asia Pacific and especially Australia's for them to ratify it."
Foreign Minister Julie Bishop, who is also in the US, held talks with representatives from both Clinton and Trump camps while in Washington on Friday (Saturday AEST) for an oceans conference, urging them to consider the deal.
