He said New Zealanders had to be careful not to run the country down with research which "might not be factually correct".
"I think you've had one or two academics in New Zealand who have presented a view and I think as the Herald editorial pointed out some of them might have been factually incorrect."
The Herald editorial today argued that Massey University senior lecturer Mike Joy's criticism of New Zealand environmental health - which was reported by the International Herald Tribune - was overstated.
In an interview with the Tribune, which was owned by the New York Times, Dr Joy cited an international study which showed New Zealand ranked 18th-worst out of 189 countries for its preservation of natural surroundings.
The peer-reviewed study in the PLoS One journal ranked New Zealand 47th-worst overall for all environmental indicators.
The Tribune article also referred to Ministry for the Environment figures which showed that more than half of the New Zealand's waterways were unsafe to swim in due to faecal contamination.
Mr Key said critics' attention had focussed narrowly on the most polluted waterways.
"The vast bulk of New Zealand waterways are safe to swim in, so when they look at those statistics they look at the worst ones, they don't measure all of them, and they measure them at the worst possible time."
The BBC also quoted Dr Joy in May 2011 when Mr Key was challenged on New Zealand's environmental record.
Dr Joy, a freshwater ecology expert, came under pressure last week from high-profile lobbyist Mark Unsworth, who slammed him in an email for undermining New Zealand's tourism industry.
The New Zealand Association of Scientists backed the Massey University expert yesterday.
Spokesman Shaun Hendy said: "The clear statement is that the potential damage to New Zealand's reputation, and economic benefit of "big-spending American tourists" outweighs the need for truth in public debate. This is an issue that the association takes very seriously."
The heightened attention on New Zealand's environment was the result of a tourism campaign which described the country as the "the real Middle-earth" ahead of The Hobbit premiere in Wellington.