The last time he was in London, the Prime Minister went on a television programme called HardTalk, where he faced an interview on the sort of questions that could be obtained from a quick search of critical websites on New Zealand. He had to defend, among other things, the "100% Pure" tourism slogan against some water quality figures a researcher had found.

John Key handled it as well as anyone could, assuring his host that if he went to New Zealand he would find it as clean, green and pristine as he probably expected. But opposition parties in Parliament have taken up the suggestion that the slogan is some sort of indictment of our environment. Answering them last week, the Associate Minister of Tourism, Jonathan Coleman, said the "100% Pure" campaign was "aspirational". This is getting silly.

The slogan is not aspirational, at least in an environmental sense. If we aspired to keep every waterway in New Zealand 100% pure we could do very little here. Effective marketing slogans, as this one is, can seldom be taken literally. They need to convey an essential truth, not a precise one. New Zealand, in the eye of anyone who has seen the world, is notably clean, green, fresh and wholesome. Tourists lured by the slogan are unlikely to be disappointed.

It is true that New Zealand does not have to try very hard to be clean, green, fresh and wholesome. With only 4 million people in an area as large as the United Kingdom, and lying in temperate latitudes, surrounded by oceans, exposed to the trade winds, we are assured of fresh air, frequent blue skies, bright light, plenty of rain, abundant vegetation and wide open spaces.

That was the country's image in tourist markets long before the state's promotion agency devised the 100% Pure brand. Like many good marketing devices it reinforced an image rather than changing anything. And like the best brands, it reflected natural advantages rather than improbable aspirations.

Yet ever since it was conceived, environmental purists have tried to co-opt it for their aspirations. The damage that dairy farming and fertiliser may be doing to aquatic life in creeks, or urban stormwater run-off may be doing to rivers and harbours, is hardly on a scale to worry tourists. If any visitors were put off coming here by those who challenge the 100% Pure brand, they would be needlessly deprived.

It is not as though these concerns are being ignored. Last month the Environment Minister found some additional money in his portfolio for cleaning up contaminated rural waterways and issued a national policy statement on freshwater management for the guidance of district councils. Its wording was not as strong as environmental groups had hoped, but nor does the pollution sound as damaging as they claim.

The principal damage they cite is to biodiversity in the waterways, which would seem sufficiently well served by the national instruction to maintain quality standards "overall" rather than strictly in every creek.

The Green Party, which hopes to make water quality the main thrust of its election campaign this year, wants resource consents to be required for any "intensified land use". That sounds like a farming straitjacket.

An agricultural economy has a clear interest in ensuring water supplies to stock and horticulture remain ample and clean. Environmentalists are a long way from convincing the country that rural waterways are degraded in ways that really matter.

They earn their cause no credit by invoking tourism's marketing brand. It is simple, innocent, evocative and - unless we are to be impossibly pure - it is true.