It has been a classic summer in New Zealand: hot days, barbecues on the beach and lazy afternoon games of cricket. But dozens of beaches, rivers and lakes have been closed to the public owing to pollution from farming.

According to a recent poll, water pollution is now New Zealanders’ number one concern: 82% of respondents said they want tougher protections for waterways, ranking it as a priority above the housing crisis, the rising cost of living and child poverty.

Martin Taylor, the chief executive of Fish & Game New Zealand, said the survey showed people were fed up with the failure of successive governments to tackle the pollution problem. “Kiwis are extremely worried that they are losing their ability to swim, fish and gather food from their rivers, lakes and streams,” he said. “People see those activities as their birthright, but over the last 20 years that right is being lost because the level of pollution in waterways has increased as farming intensifies.”

According to the environment ministry, two-thirds of all rivers are now unswimmable and three-quarters of New Zealand’s native freshwater fish species are threatened with extinction. New Zealanders are now encouraged to check their local council websites for public health warnings before heading out for a day on the water.

Declining water quality has coincided with a boom in New Zealand’s dairy industry – it is the largest exporter of dairy in the world. Cow effluent and fertiliser run-off are significant polluters of inland waterways, as are beef, sheep and deer farming. Mass deforestation and the extensive clearing of native wetlands has also played a significant role.

According to the National Institute of Water and Atmospheric Science (Niwa), there is no doubt that the growth in pastoral farming, particularly dairy farming, is the main culprit for declining river water quality over the last 20 years. Rivers and streams in dairy country such as the Waikato, Southland and Canterbury are the most polluted in the country, Niwa says.

Dairy stock require rich, green pastures to produce the best milk, which has led to a proliferation of irrigation and fertiliser use. As the dairy herd has increased, their nitrate-rich effluent has flowed into the waterways, asphyxiating ecosystems, causing toxic algae blooms, making indigenous food sources inedible and making it unsafe for people to swim in, drink or sometimes even touch the water.

In 2016, four people died and 5,000 fell ill as a result of a gastroenteritis outbreak in the North Island town of Havelock North. The probable cause was sheep faeces contaminating the town’s water supply, a government inquiry found.

“I really thought that once people started dying, that things would change and people would take notice. However, that doesn’t seem to be the case,” said Prof Russell Death, an ecologist, in a keynote address at the New Zealand Freshwater Science Society’s conference last month.

“It still seems to me that water quality is not improving. If it is, it’s only very, very slowly improving. To my mind, there’s no convincing evidence that we’ve turned around the decline that’s been going on over the last 20 years.”

Facebook Twitter Pinterest ‘Birthright’: a lake, stream and snow-capped mountains near Hanmer Springs on New Zealand’s South Island. Photograph: Greg Wood/AFP/Getty Images

The Labour coalition government, led by the prime minister, Jacinda Ardern, has pledged to make the country’s rivers and lakes swimmable within a generation, describing it as a “birthright” for New Zealanders. It is a more ambitious goal than that of the previous National government, which promised only to make them “wadeable”. But action has been slow, and experts say it has taken a back seat to housing and entrenched social welfare issues.

The environment minister, David Parker, caused an outcry last year when he told a current affairs show that New Zealand had too many cows, raising the possibility of a cap on numbers. Parker has since backed away from this statement and clarified that a nutrient run-off cap is more likely. Encouraging changes to land use in favour of more cropping and horticulture is also a government priority.

“There are some people who are in denial,” Parker said in a TV interview. “Now, those people will have to be regulated to do the right thing, because they may not be willing to do it voluntarily. We’re actually not going to subsidise land use change, but we will enable it through the new technologies that we are willing to subsidise to bring forward.”

Chris Lewis, the dairy chair for the Federated Farmers of New Zealand, acknowledged that the industry was “far from perfect” and said it was keen to change, with improvements expected within the next 10 years.

Taylor, the Fish & Game chief, said legislation was needed. “While many farmers do understand the need for action and are making the necessary changes to how they use their land, there are still significant numbers who are refusing to follow their example,” he said. “They have to be made to change.”