Another danger in our drinking water has been identified, and it's affecting one in six Kiwis.
A landmark study has found that 800,000 Kiwis are exposed to levels of nitrates that are linked to an increased risk of bowel cancer.
Every week for the past two years, Mike Glover has driven up the road to collect safe drinking water for his family.
You can't see it or taste it, but his tap water in Selwyn, Canterbury, has five milligrams a litre of nitrate-nitrogen.
"Studies suggest it's way higher above a level that will potentially cause cancer," Glover told Newshub.
"It's just not worth the risk."
Five milligrams is within 'safe limits' in New Zealand.
But even much lower levels have been linked to bowel cancer in at least seven international studies.
So two years ago, Glover turned off his tap.
"It's only going to get worse, there's a hell of a lot more nitrate to come down," he said.
A testing site at Lincoln University found others in the same boat, with many people testing at similar or higher nitrate levels to Glover.
Nitrate fertiliser speeds up plant growth - and is used in intensive farming systems.
It can get into waterways with rain, irrigation and through animal urine.
Studies show 95 percent of the nitrates in fertilised grass ingested by animals ends up urinated back on to the ground.
Freshwater ecologist Mike Joy says it's a big part of the problem.
"This is one of the consequences of it - is nitrates in the water," he told Newshub.
Joy co-authored New Zealand's first study into nitrate levels in our drinking water.
Jayne Richards is another author, and she collected data from water suppliers right across the country.
Their research reveals about 800,000 Kiwis - or one in six of us - are drinking water polluted by nitrate levels that can increase the risk of getting colorectal cancer.
A map of New Zealand's cattle density and nitrate levels shows the higher the density of cows, the higher the level of nitrate pollution.
"We have the highest rates of colorectal cancer in the world, in NZ, and the highest rates within NZ correlate to where those high nitrate levels are in groundwater and drinking water," Joy said.
Otago University health researcher Tim Chambers is another co-author.
"It's one of the first studies to try and pull together a nitrate contamination data set and look at this issue. It's concerning because we do have high levels of nitrates in our drinking water, and it's likely to get higher with dairy intensification."
A Danish study monitoring 2.7 million people over 23 years found a significant increase in bowel cancer risk at just 0.87mg/litre of nitrate-nitrogen per litre of water. At 2.1mg/litre there was a 15 percent increase.
New Zealand's drinking water standard is more than five times that - at 11.3mg/l of nitrate-nitrogen.
It's estimated elevated nitrate levels could be responsible for nearly 300 bowel cancer cases here every year.
"You don't wait until a whole lot of people have cancer, you go 'well here's the evidence, let's prevent this from happening and put a tough limit on it'," Joy said.
For now, the Ministry of Health regulates what comes out of your tap, and it says standards for our water are in line with the World Health Organisation.
This year the ministry's power is being handed to a new authority - Taumata Arowai. And the Water Services Bill, which is currently before Parliament, will enable Taumata Arowai to issue or adopt drinking water standards.
The study authors say it's the perfect opportunity to toughen up those laws around nitrate levels.
"It's about getting those new pieces of legislation right, because they really are a once in a generation opportunity," Chambers said.
The authors want new laws to limit cattle density and the use of synthetic fertilisers, so that the nitrates don't get onto the land and into the water in the first place.