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Prime Minister John Key has defended the credibility of New Zealand's clean, green image, maintaining the country is "100 per cent pure".

Mr Key was responding to BBC Hardtalk reporter Stephen Sackur, who suggested the country's slogan is no longer true.

"Dr Mike Joy, of Massey University, a leader environmental scientist in your country, just the other day said 'we are delusional about how clean and green we are'," Mr Sackur said.

He cited Mr Joy's research, which found half of the lakes and 90 per cent of lowland rivers in New Zealand are polluted.

Mr Key, who is also the Minister of Tourism, said he does not share Mr Joy's view.

"He's one academic and, like lawyers, I could provide you others who would give a counter view," he said.

"If anybody goes down to New Zealand and looks at our environmental credentials and looks at New Zealand, then for the most part, I think in comparison with the rest of the world, we are 100 per cent pure."

Mr Sackur responded New Zealand was clearly not "100 per cent pure" as it has problems with water pollution and the decline of native species. He again cited Mr Joy, who has said New Zealand has become complacent about its environmental reality.

"I don't actually totally agree with that proposition," Mr Key said.

"For a start off, yes of course the population is getting larger and that creates some form of pollution and yes we have a large agricultural base as we have been intensifying our dairying operations - that's had some impact on our river quality.

"Now what the Government has been doing is demanding much higher environmental standards. So actually for the most part, you jump in any New Zealand river or stream, you breath the air and you walk up a mountain, I'd argue with you that if you don't believe it is clean and green then you need to show me a country that is cleaner and greener."

However Green Party co-leader Dr Russel Norman said New Zealand needed to "get real" about cleaning up the country's rivers and lakes, as the world is realising the country does not live up to its "clean green image".

"The clean green New Zealand brand is worth $18.4 billion, but this asset is at risk unless we take immediate action to restore our waterways. To be effective in the long-term, our brand must reflect reality," Dr Norman said.

Business New Zealand chief executive Phil O'Reilly admitted the country faced some real environmental issues, but said Dr Joy's article was full of hyperbole.

He told Newstalk ZB the clean green issue was actually much more complex than simply talking about the environment.

Mr O'Reilly said it was important to have a complex and sensible discussion around that.