NICK SMITH may be the first politician to be immortalised in horse manure. Before the recent general election, a super-sized sculpture depicting the environment minister, trousers down, squatting over a glass, was paraded through central Christchurch. It was carved from dung in protest at an alarming increase in water pollution. Data published in 2013 suggested that it was not safe for people to submerge themselves in 60% of New Zealand’s waterways. “We used to swim in these rivers,” says Sam Mahon, the artist. “Now they’ve turned to crap.”

Mr Smith’s National Party is now out of government. But the real villains behind New Zealand’s deteriorating water quality are still at large: cows. Scrub where sheep once grazed is being given over to intensive dairy farms—some of them irrigated to help the pasture grow. Some 6.6m cattle are now squeezed into the country of 4.7m people, transforming even an iconic arid grassland, the Mackenzie Basin (made famous by the “Lord of the Rings” films), into a tapestry of emerald fields.

The first concern is bovine urine, which is rich in nitrogen. Nitrogen can cause toxic algae to grow when it leaches into water. Nitrogen fertiliser, used to increase fodder yields so that more cows can be raised on less land, exacerbates the problem.

At many of the sites where the government tests the groundwater it contains too much nitrate to be safe to drink—a particular problem in New Zealand, since water in much of the country has long been considered clean enough that it is used as drinking water with only minimal treatment. In Canterbury, one of the most polluted areas, expectant mothers are told to test tap water to avoid “blue baby syndrome”, a potentially fatal ailment thought to be caused by nitrates. The poisonous blooms have killed dogs.

An even greater concern for human health comes from cow dung, which contains nasty bacteria such as E.coli. Three people died last year after a well was contaminated with another bug called campylobacter. Sheep were to blame in that case, yet cows have a proclivity for wading in rivers and their faeces often find their way into water. New Zealanders are twice as likely to fall ill from campylobacter as Britons, and three times more than Australians or Canadians.

And then there is the damage to native flora and fauna. The algal blooms suck the oxygen from rivers. Sediment washed from farmland can also choke the life out of streams. Almost three-quarters of native species of freshwater fish are under threat.

New Zealand is a rainy place, but farmers are also criticised for causing rivers to shrivel and groundwater to fall in certain overburdened spots. One recent tally suggested that just 2,000 of the thirstiest dairies suck up as much water as 60m people would—equivalent to the population of London, New York, Tokyo, Los Angeles and Rio de Janeiro combined. Most is hosed on the stony Canterbury region, including the Mackenzie Basin. Earlier this year locals were forced to rescue fish and eels from puddles which formerly constituted the Selwyn river, after drought and over-exploitation caused long stretches to dry up.

Dairies are trying to clean up their act. Farmers have fenced off thousands of kilometres of rivers to prevent livestock from wading in. Some have planted trees along waterways to curb erosion; others remove animals from muddy fields during winter. Some parts of the country are using more sophisticated techniques: around Lake Taupo, the country’s biggest lake, farmers can buy and sell nitrogen allowances in a cap-and-trade scheme. A technique called “precision irrigation” may curb both water consumption and the leaching of nitrogen.

Earlier this year the National Party launched a plan to make 90% of rivers “swimmable” by 2040. Yet it ignored several recommendations of a forum of scientists and agrarians established to thrash out water policy, and removed elected officials from an environmental council in Canterbury after they attempted to curb the spread of irrigation. One of its big initiatives to improve water quality involved lowering pollution standards, making rivers look much cleaner at a stroke.

The Labour Party, now in government, had promised during the election campaign to tax irrigators and use the cash to clean up rivers. But Labour’s populist coalition partner disliked the idea, so it has been dropped. Jacinda Ardern, the new prime minister, says that she will charge companies that bottle and export local water—little more than a gesture, as they account for only a tiny share of water use.

Environmentalists argue that the national dairy herd should be cut to prevent further damage. That may not be as hard on farmers as it sounds, argues Jan Wright, a former parliamentary commissioner for the environment. She says recent growth in the industry has been relatively inefficient, denting margins. Yet the chances of change are slim. The regulations governing Fonterra, a big dairy co-operative, encourage volume more than value, says Kevin Hackwell of Forest & Bird, a pressure group. And pollutants moving through groundwater can take decades to emerge in lakes. The worst may still be to come.