A large North Canterbury irrigation scheme is being allowed to exceed recommended levels of nitrate in the water, potentially for years.
That's despite the known risk that it could cause blue baby syndrome.
The Amuri Irrigation Company's (AIC) scheme covers about 28,000 hectares in Hurunui and Waiau and mainly irrigates pasture.
The regional council is letting the scheme have excessive nitrate levels because Amuri is transitioning into a more efficient irrigation system.
But health officials and environmental advocates argue that's not good enough.
The scheme is required to test wells every three months for nitrate levels and if the nitrate is above the maximum acceptable level of 11.3 grams per cubic metre in a drinking water well. The homeowner must then be notified because of the risk to young babies and pregnant women.
In the last year, the nitrates have been excessive twice at two separate drinking wells, one bore tested 14.3 grams in June and another tested 15 grams in August.
Despite the risk to health, there is no penalty for this because it is allowed within the resource consent.
Forest and Bird freshwater advocate Annabeth Cohen said this was just one example of the type of polluting that was happening all over Canterbury with no consequences from the regional council.
"There appear to be no limits to the amount of pollution going into the system. We want them to indicate that they're going to stop the pollution and clean it up, or hold AIC accountable to clean it up."
Amuri said there were no babies or pregnant women at the homes whose wells tested excessive levels of nitrogen.
Canterbury's medical officer of health, Alistair Humphrey, said nitrates were a small but significant risk to small babies.
In the United States there are about half a dozen cases of blue baby syndrome a year, where the blood stops carrying oxygen and it could be fatal.
The last case in New Zealand was in 1994, but nitrate levels have steadily increased since then.
Dr Humphrey said, as a result, the DHB has had to enlist midwives and GPs to ensure women were aware of the risk from nitrates in risky areas.
"It's a bit like when a woman books for her pregnancy she will get an ultrasound scan and various blood tests, we're basically asking that they have their water tested at the same time if they have their private bore in a moderate or highly risky area."
Dr Humphrey said families were now carrying the cost of having to fix their water supplies because of pollution and cites an example north of the Waimakariri where 10 years ago people were able to drink their bore water.
"They are looking out of their kitchen windows at hundreds and hundreds of dairy cattle and they can't drink their water anymore - 10 years on they now have to get their water from elsewhere.
"So, I don't think it's fair to those families, that in some cases have lived in those houses for generations, to expect them to go off and buy expensive iron exchange systems to fix their water."
Dr Humphrey said there should be consequences for when farmers exceed the acceptable nitrate level, even if it was not a breach of consent, and the farms that were doing the contamination should pay.
But the regional council's Andrew Arps said the excessive nitrate levels were allowed because Amuri is transitioning into a more efficient irrigation system.
He couldn't give a deadline when the company was no longer allowed to exceed the nitrate limit.
"Well effectively it's part of that transition from one scheme to the other and the other tools that we have in place around driving down the nitrogen levels - that's over a 10 to15 year period - we may well see them happening earlier but those things take time to come through."
Mr Arps said Amuri had reduced nitrogen across the catchment and it did face the penalty of having to notify people if their bores have excessive nitrate levels.
Amuri chief executive Andrew Barton said the nitrate levels were a focus for his company.
"Our focus is on trying to improve water quality and that's surface water and groundwater so from our perspective it's an area that concerns us as well as the public in general and we are putting effort and resource into trying to improve water quality."
Dr Humphrey said a recent Danish study, which linked nitrates to colon cancer, threw more light on the risk from nitrates and more research needed to be done in this area so the risk to New Zealanders from nitrates in groundwater was truly assessed.