John Key said there was no doubt Mark Zuckerberg (pictured) was a generous philanthropist, who had pledged to give away his fortune, for which he should be commended.

Prime Minister John Key says he has delivered a blunt message to Facebook founder Mark Zuckerberg that the social media firm has a public relations problem over not paying its fair share of tax.

Key talked to the billionaire businessman on the margins of the Apec summit in Peru during a conversation that included Canada's Prime Minister Justin Trudeau and Mexican President Enrique Nieto.

"I was reasonably blunt. I said I thought Facebook did have an issue in terms of its global tax, with the perception of its tax policy. And I thought he needed to change that," Key said.

"It wasn't so much whether they paid their fair share of tax in New Zealand ... Maybe they do maybe they don't - I can't assess that."

Key said as a company Facebook had a public relations issue and should think about how to resolve that, or demonstrate to the world that they did pay their fair share of tax.

He said Zuckerberg "was a bit surprised, but I don't think i'm doing him any favours not telling him that".

John Key talked to the billionaire businessman on the margins of the Apec summit in Peru.

Key said there was no doubt Zuckerberg was a generous philanthropist, who had pledged to give away his fortune, for which he should be commended.

But it was not about generosity.

"There's a bit of a feeling - that would have got Donald Trump elected - that somehow the world wasn't fair."

Key said he thought Facebook could see the merits of the arguments.

But he said Facebook may pay its fair share of tax: "I don't know, but I do know you've now got the OECD asking all these countries to hold hands together at major tax reforms when it comes to global multinationals, and at the forefront of that, one of those ... is Facebook."

One day the users would wake up and ask "why they had to pay tax if this company is not going to," Key said.