Jacinda Ardern explains why she didn't laugh at Donald Trump's speech

Leaders of the world's nations united in laughter at the United Nations this morning after President Donald Trump made an incredible claim about his presidency.

"In less than two years my administration has accomplished more than almost any country in the history of our country," Mr Trump said, to some giggling.

"So true," Mr Trump said, causing laughter to spread through the chamber.

"Didn't expect that reaction, but that's OK," he said.

But the Prime Minister Jacinda Ardern wasn't among those laughing - and afterwards, she told reporters why.

"I didn't laugh. I was listening to the words being spoken... The President made a statement around his view around his success relative to past presidents," she said.

"For me, I was respecting the fact he had the floor and what he had to say."

Mr Trump later told reporters he had made an intentional joke.

"That was meant to get some laughter, but it was great," he told media.

Mr Trump's speech emphasised his 'America first' agenda, which is almost the polar opposite of the message Ms Ardern is taking to the UN - of multilateralism and obeying the rules-based order.

"There wasn't anything in President Trump's statements today that surprised me," Ms Ardern said.

"He obviously takes a very particular view on issues around sovereignty, multilateralism and, of course, issues around Iran."

While Ms Ardern wasn't laughing at Mr Trump, the leaders' spouses had a parallel meeting of straight faces when Clarke Gayford tripped over a flag in front of Mr Trump and his wife Melania Trump.

Ms Ardern said Ms Trump reassured Mr Gayford and told him not to worry about it, Stuff reports.

Ms Ardern said Mr Trump congratulated her on the birth of Neve, and Ms Ardern raised the issue of tariffs on steel and aluminium.

National Party leader Simon Bridges said the laugher at Mr Trump's claim was "unfortunate".

"Clearly, Mr Trump is a divisive person who polarises, in a sense. He's certainly got a strong perspective. There's some things he's doing I agree with. There's a bunch of other areas where he is divisive."

"President of the United States - he deserves some respect. I certainly wouldn't have been laughing," Mr Bridges said.

He said parts of Mr Trump's tax agenda have had a simulative effect on the economy.

Mr Trump's Tax Cuts and Jobs Act cuts corporate tax from 35 percent to 21 percent, and cuts income tax almost across the board.

While the US economy is performing well and unemployment is low, those tax cuts are projected to add billions to the United States' deficit. The New York Times reports the United States' spending on interest will outstrip spending on the military in a decade. 

Ms Ardern is due to speak at President Macron's Global Planet Summit on Wednesday afternoon (local time), before appearing on the Late Show with Stephen Colbert.