The race for the White House has been the closest and most unpredictable election in American history, a former Australian ambassador to the US says.

As Kamala Harris and Donald Trump make their final pitches to voters ahead of the election on Wednesday, Australian time, former ambassador Joe Hockey said most Americans had already made up their mind on who they want to run the country.

With polling still showing a tight race for the presidency, Mr Hockey said it was possible it would take days after the election to determine who would win the vote.

"Unlike Australia, where we have compulsory turnouts and elections here, it is a mystery how many new voters will come out and how they're going to vote," he told ABC's Insiders program on Sunday.

"It's going to come as a surprise to Australians, given there are so many polls that we're in a position where we can't pick the winner, but the fact is, no one knows how big the turnout is going to be.

"It is the most unpredictable and closest election in American history, and no one knows the result. No one knows the implications of it with any certainty."

More than 70 million Americans have already cast their ballot ahead of election day, while the outcome set to come down to just a handful of swing states.

(Former Australian ambassador to the US, Joe Hockey, says the election is going to be close. (Dean Lewins/AAP PHOTOS)

Mr Hockey said a second Trump presidency would not be different to that of his original term.

"Donald Trump 2.0 won't be a hell of a lot different to Donald Trump 1.0, and that's good for us," he said.

"It offers some measure of predictability, whereas Harris will be a little less predictable because she needs to differentiate herself from the Biden presidency."

Education Minister Jason Clare assured the relationship between Australia and the US would remain the same, regardless of which party wins office.

"If the polls are right, it's likely to be a very close result. I think whatever happens, it would be good if it's a clear result for the United States," he told Sky News.

"The United States is our closest ally, and that is a matter of bipartisan support ."

Opposition Foreign spokesman Simon Birmingham said the government needed to work with whoever comes into the White House.

"I hope the Albanese government has got comprehensive plans there for how they ensure that Australia's interests are safeguarded," he told Sky News.

"It's critical that as strong as our relationship is, we don't take any of those equities for granted in the way in which we engage with whoever the next US, president and their administration may be."

Senator Birmingham said it was critical bipartisanship around the AUKUS partnership, which will have Australia acquire nuclear submarines from the US, continues.

Mr Hockey said advocacy in the US on the submarine deal would remain the same.

"Donald Trump sees Australia very favourably when it comes to issues like national security. I think that will flow through to AUKUS, that he won't want to change the relationship with AUKUS," he said.

By Andrew Brown in Canberra