Freshwater ecologist Mike Joy says he has the science to disprove New Zealand's "clean, green" image, and he has found holes in a study the Government has used as a counter-argument.
Dr Joy's speech to Forest & Bird today will also outline his case for why the "100% Pure" advertising campaign is misleading.
The issue came to attention last month, when Prime Minister John Key was grilled on BBC's Hardtalk programme by host Stephen Sackur, who used Dr Joy's data to suggest the slogan was no longer true.
"Well, that might be Mike Joy's view, but I don't share that view," Mr Key said at the time. "Like lawyers, I can give you one that will provide you with a counter-theory."
After the show aired in New Zealand, Mr Key faced a barrage of questions in Parliament from Green Party co-leader Russel Norman, who suggested he was playing down the environmental impact of intensive farming.
Mr Key said the impact was "not great" and pointed to a 2010 study from Columbia and Yale universities that ranked New Zealand second only to Iceland in terms of water quality, with a score of 99.2.
But Dr Joy poured cold water on the study yesterday, claiming the data it used was "totally flawed". Firstly, out of the 130 countries ranked, only half actually had any water quality data available for the comparison, while the rest had estimates.
The data was also an unfair representation of nations as a whole. "The 11 sites they got from Australia were all from one state. You can hardly represent the water quality of the whole country by examining one tiny portion of it."
He said half of the 80 measurements used to calculate New Zealand's water quality were from pristine "control sites" known to be free from the impacts of agricultural and industrial pollutants.
Mr Joy said he called the Yale study's authors when he heard it being quoted by the Government and was "shocked" by the research it was based on.
When asked yesterday if he still stood by his comments, a spokeswoman for Mr Key said: "The prime minister does not share the view of Mike Joy, and has no further comment to make."
- © Fairfax NZ News
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