New Zealand Prime Minister John Key has likened the country's "100% Pure" brand to a marketing campaign by hamburger giant McDonald's - and says no-one expects it to be 100 per cent true.

"It's like saying 'McDonald's, I'm loving it' - I'm not sure every moment that someone's eating McDonald's they're loving it ... it's the same thing with 100% Pure. It's got to be taken with a bit of a pinch of salt."

Key made the comments amid controversy over criticism by Massey University scientist Mike Joy of New Zealand's clean, green brand.

Joy's criticisms were printed in the New York Times ahead of a major push by Tourism New Zealand around the Hobbit movies, under a deal negotiated by Key as tourism minister with movie bosses.

Joy told the newspaper the reality of New Zealand's environmental record came nowhere close to matching the 100% Pure brand. His comments have sparked a backlash from, among others, lobbyist Mark Unsworth, who accused him in an email of an ego trip.

Key said even cavemen failed to live up to the "100% Pure" standard Joy wanted. "If ... we should be 100% Pure and ... there's no economic activity ... cavemen burning fires has a environmental impact."

- Fairfax NZ News/Stuff.co.nz