Greenpeace is urging regional councils to lower the limit of allowable nitrates in groundwater, which they say poses a cancer risk.
Water testing of locals' bore water found Mid-Canterbury's water was "loaded" with nitrate contamination, according to the group.
The current limit in New Zealand follows the World Health Organization guidelines, for 11.3mg per litre of water.
But Greenpeace campaigner Steve Abel said recent studies show an increased bowel cancer risk when nitrates are above 1 milligram per litre, and an increased risk of pre-term birth.
The environmental group offered free water testing in Ashburton and tested over 100 samples.
Abel said over 90 percent of the samples from the dairy farming-heavy town were above 1 milligram, many of the locals were shocked and worried at the results.
"It's difficult and costly to remove nitrate from drinking water, the best thing obviously is to stop it from getting in there in the first place.
"But, once again the onus falls on the owners of the bores to pay for that cost even though they themselves did not cause the contamination."
Greenpeace, Forest & Bird, the Environmental Defence Society and Choose Clean Water have called for the government to lower the level for nitrate water contamination to be lowered from 11.3 to 0.87mg per litre of water.
They want the levels addressed by a combination of phasing out synthetic nitrogen fertiliser, lowering cow stocking rates and supporting farmers to farm organically.
"Everyone should be able to trust that the water from their tap is clean and safe," Abel said.
"Unfortunately, nitrate contamination is a growing concern for people in areas with intensive dairying, such as Canterbury, Southland and Waikato. Bore owners in these regions deserve to know what's happening with their water.
"Local and central governments need to act fast to ensure safe drinking water for our communities by phasing out synthetic nitrogen fertiliser, lowering cow stocking rates and supporting farmers to shift to regenerative organic farming."
A task force is being set up by the Ministry of Health to investigate the link between nitrates in drinking water and cancer risks in New Zealand, and whether a review of the drinking water standards is needed.
This week Waikato Regional Council scientists said a study into the issue has revealed rising nitrate levels in wells and rivers across the region, and a growing body of new international research that should be reviewed to consider the implications for safe drinking water.