Donald Trump's lewd comments about women are "indefensible", Prime Minister John Key says.

Speaking shortly after Trump squared off against Democratic nominee Hillary Clinton in the second presidential debate, Key condemned Trump's comments, captured on video in 2005 and released over the weekend.

"I'm not going to critique it all, you'll have to draw your own conclusion. My personal view is that language is indefensible," Key told reporters at his regular post-Cabinet press conference.

"The comments themselves are indefensible. That's why he's come out and apologised for them. And I think all of us that have had a chance to see Donald Trump over the last few months would realise that he's not a person that is prone to apologising very often."


Asked if Trump was fit to be President, Key declined to give his opinion, saying it was a matter for US voters.

He said he only caught a few minutes of today's town hall-style debate at Washington University.

The exchanges he saw reflected it was an "intense" part of the campaign and also the controversy over the Trump video, Key said.

In that 2005 video, Trump is heard discussing his attempts to bed a married woman, and bragging that he can "do anything" to women because he is a celebrity.

"When you're a star, they let you do it. Grab them by the pussy. You can do anything," he says.

Meanwhile, another video clip of Trump talking about women has emerged - this one during his visit to New Zealand in 1993.

In the TV3 footage, Trump is asked if he enjoyed the image people have of him as "this high-rolling tycoon associated with glamorous women".

"No, I don't enjoy that image. I guess I have that image," he responds.

"I think women are beautiful, I think certain women are more beautiful than others, to be perfectly honest. And it's fortunate I don't have to run for political office.

"But...frankly it's not the image I want. The image I want is that I'm doing great in business."