Those who think Donald Trump will blow today's presidential debate should think again, Prime Minister John Key says.

"I tell you what, having been in quite a lot of them myself, usually they favour the underdog," Key said of the 90-minute debate, due to start 2pm New Zealand time.

"There will be a lot of people that might not expect too much from Donald Trump - they'll say that he tends to have a fair bit of rhetoric and not much else. But as he proved in the Republican process, he came through and won those."

Today’s televised debate between Hillary Clinton and Donald Trump will be like no other in the history of American politics. While no previous candidate has the temperament or unpredictability of Trump, here’s a look back at some of the other more memorable presidential debates.

Asked if he was picking Trump to outperform Democratic nominee Hillary Clinton, Key said he wouldn't go that far.

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"But what I would say is that he might do better than people expect.

"They are saying 75 million people are going to watch it, so it's going to be quite interesting."

Much of America and millions around the world are expected to watch the first of three debates, billed as one that could make or break both campaigns.

A Washington Post poll released Sunday shows likely voters split nationally 46 per cent for Clinton and 44 per cent for Trump, with Libertarian Party nominee Gary Johnson at 5 per cent and Green Party nominee Jill Stein at 1 per cent.