Health ministry launches review into effects of nitrate in water

The Ministry of Health has quietly launched a review into the health effects of nitrate in drinking water.

It comes as campaigners renew calls to slash nitrate limits following Denmark's newly-formed coalition government agreeing to drastically lower its limit.

The coalition agreement between Denmark's governing parties calls for the drinking water nitrate limit to be cut almost 90 percent, from the equivalent of New Zealand's 11.3 milligrams of nitrate-nitrogen (NO3-N) per litre to 1.3mg/L.

New Zealand's 11.3 milligrams nitrate-nitrogen per litre maximum acceptable value (MAV) is based on 1958 World Health Organisation guidelines to prevent babies dying from blue baby syndrome (methemoglobinemia).

Advocates have long argued this limit does not protect from other potential health effects, including colorectal cancer and premature birth.

Public health researcher Professor Tim Chambers said a similar drop in New Zealand would affect dozens of public water supplies and many more private wells.

About 800,000 people could be on water supplies with nitrate above 1 milligram per litre, according to 2021 research from Otago and Victoria University.

GNS Science estimated in 2025 that more than 21,200 people could be drinking water contaminated with nitrate above legal limit, while more than 100,000 people could be drinking water above half the MAV (5.65 milligrams per litre).

A spokesperson for Health Minister Simeon Brown said New Zealand's current limit was consistent with international guidelines, including the World Health Organization, the European Union and Australia.

The responsibility for ensuring safe drinking water lay with the suppliers - councils or private well owners, the spokesperson said.

Taumata Arowai head of operations Steve Taylor said nitrates would be included when the water regulator carried out its usual five-yearly review of drinking water standards next year.

But the Ministry of Health has revealed it began a technical review months ago into the health effects of nitrate in drinking water.

A spokesperson said the internal review began in February and was expected to be completed by the end of the year.

The ministry had not publicly announced the review because it was "business as usual", the spokesperson said.

"This is a literature review which will consider publications from January 2021 - February 2026, focusing on the human health effects from nitrates in drinking water. Animal studies may be considered in certain instances where there is no evidence in humans for the included health outcomes," the ministry said.

"The Ministry of Health continues to review emerging evidence and research to better understand the link between nitrates and adverse health outcomes. Following on from the Danish study and their recommendations, the Ministry of Health has initiated a technical review of the human health effects of nitrates in drinking water. The results of this work will be provided to Taumata Arowai to inform any future review of the permitted MAV levels."

While nitrates in urban supplies tended to be very low, the ministry was "aware there are some rural water supplies where the nitrate level has at times been higher than the maximum acceptable value".

The ministry advised the public to "raise their concern directly with their drinking water supplier" if they believed their drinking water was not safe.

Chambers said Denmark's move to slash the limit followed growing evidence showing increased health risk at lower levels, particularly the risk of colorectal cancer.

It was too early to know what the Ministry of Health's review would accomplish because there was little information on its parameters, what studies would be included and what would be done with the findings, he said.

However, it had long been unclear what work the ministry was doing to review nitrate guidelines, so confirmation they were taking action to review the literature was welcome, Chambers said.

But he called for transparency.

"I trust the Ministry of Health to be able to conduct a review of the standard as long as the terms of reference for that review are transparent and public. You can assess the rigour of the review if we're able to assess the methodology they're using to conduct the review."

Greenpeace agriculture campaign lead Amanda Larsson said she was encouraged by the minstry's review.

"The evidence is clearly strong enough that the Danish government feels compelled to lower the nitrate limit, so I would hope to see New Zealand following suit when it reviews the same science," she said.

There was a lot of concern in rural communities about rising nitrates in drinking water particularly for those on private bores, who had to test and treat their own water, Larsson said.

Denmark has the most pigs per person of any country and has struggled with the effects of intensive pig farming.

Environmental, health and animal welfare impacts took centre stage in March's election.

The Danish coalition agreement, finalised last week after two months of discussions, included a commitment to follow the recommendations of an international working group commissioned during the government's last term.

In late 2025 the group recommended limiting nitrate in drinking water to the equivalent of 1.35 milligrams of nitrate-nitrogen per litre of water.

New Zealand's limit was consistent with World Health Organization (WHO) guidelines.

But Chamber said relying on WHO guidelines was problematic as the organisation dealt with guidelines for hundreds of contaminants.

"The problem is the WHO has a lot of different water quality parameters they're interested in and limited resources, so they prioritise which contaminants they're going to review at each wave. The last time they did nitrate was in 2016 and the time before that was in 2003, so there's roughly 15 years between reviews."

It was unlikely the WHO would review nitrate again in the next five to ten years without substantial pressure from member states, he said.

Other measures could be introduced ahead of a decision to lower the nitrate limit, such as regional councils regulating for lower limits in hot spots.

However, that was not a silver bullet, Chambers said.

"We're still consenting conversions of new dairy farms in the Canterbury region after declaring a nitrate emergency," he said.

The Canterbury Regional Council, which has consented capacity for up to 25,000 more cows in the region by 2027 since a temporary ban on dairy conversions expired in January 2025, had far lower nitrate limits for some areas in its land use plan, he said.

"They have limits that say if a drinking water source reaches over half the MAV, that's the limit that they're willing to accept, but when that's breached, no action is actually triggered," Chambers said.

"In Waimakariri that's happened with the Oxford rural supply, where the level of the Oxford supply has gone over half the MAV and nothing has happened."

It would not be difficult for much of the country to meet a drastically lowered standard, but for the worst affected areas rising nitrate levels could be costly, he said.

"If you're reliant on water suppliers to try and meet that MAV, it's going to be financially impossible for many to try and install nitrate treatment at the treatment plant."

Protecting water sources and changing land use was the easiest and most affordable way to reduce nitrate levels, he said.

Federated Farmers vice-president Colin Hurst said the group was not aware of the Ministry of Health's technical review or the Danish government's move but welcomed new information.

"We welcome the science people investing in this stuff. We need the scientists to tell us, investigate ... in a scientific method statistically done properly and tell us what is going on and then we'll understand it," he said.

"Certainly ...if over time things are indicating things are getting a little bit worse ... if that's the case, yeah, we welcome that type of information."

It was a complicated issue further muddied by the piecemeal nature of some regional council monitoring, he said.

But the science was not settled when it came to effects on human health, Hurst said.

He pointed to Bowel Cancer New Zealand's 2021 position that nitrates in drinking water were highly unlikely to increase the risk of bowel cancer and the Prime Minister's chief science advisor's 2022 report that found the evidence of health impacts was inconclusive.

But Larsson said public health scientists estimated there were about 100 cases of bowel cancer and up to 40 deaths each year linked to nitrate in drinking water.

"It's often the case the science is quite advanced before the policy catches up. We've seen that in the case of asbestos, lead and petrol, and other things where concern from public health researchers was flagged long before the policy caught up," she said.

Meanwhile, vested interests caused regulation to stall and sometimes reverse, she said.

"We're seeing the government actually weakening freshwater protections instead of strengthening them and that's because of the influence of the agribusiness lobby," Larsson said.

Denmark's move could have a domino effect in other parts of the world because of the European Union's common agricultural policy and growing concerns about nitrate contamination in other European countries, she said.

Cutting New Zealand's limit by 90 percent would significantly change land use in parts of the country, Larsson said.

But that was possible and dairy's proliferation had only occurred in recent decades.

"That's only in the last 20 years or so, and it's entirely possible to shift the way that land use happens again. That's ultimately what needs to happen in order for people to have access to safe drinking water."

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