The Canterbury Medical Officer of Health says a proposed new limit on nitrate leaching from farms in the Selwyn District is not strict enough, and could jeopardise the region's drinking water.
The restriction is part of a change to the regional land and water plan being considered by independent commissioners, which has just finished a first phase of hearing submissions.
Nitrates are produced by fertiliser and animal manure.
Environment Canterbury is proposing a nitrate limit of 8.5 milligrams per litre of water, close to the maximum safe level allowed.
With 50,000 hectares of land in Selwyn already taken up by dairy farms and another 30,000 possible if a new irrigation scheme goes ahead, Canterbury Medical Officer of Health Alistair Humphrey said the limit should be almost half of this.
"Nitrates take a very long time to increase, they take a very long time to decrease.
"So it's like turning an oil tanker around.
"You cannot wait until you get to the maximum accepted value before you act. You need to act earlier than that."
Dr Humphrey said the risks of letting nitrate levels get dangerously high are many including to new born babies who can suffer blue baby syndrome.
"If that formula is made up with water that has a high level of nitrate in it, that binds very very strongly to the baby's foetal haemoglobin.
"So it has the same effect as you and I breathing in carbon monoxide."
Green Party water spokesperson and former Ecan councillor Eugenie Sage said Government and the commissioners it appointed to run the regional council were using Selwyn as a laboratory.
She said proposed nitrate limits were more permissive than other parts of the country and could soon become the norm elsewhere if the green light is given in Selwyn.
"We shouldn't be trading away our drinking water quality for short term financial benefits from irrigation and more dairying.
"It's almost impossible to clean up nitrate-nitrogen in ground water when it's got too high. We're much better taking a precautionary approach."
She said unlike other areas in Canterbury, changes to regional plans can not be appealed to the Environment Court and appeals could only be taken to the High Court on points of law.
"That's a major erosion of sound RMA planning because normally with the environment Court you get an opportunity for cross-examination and a very robust consideration of all of the issues."
A Dunsandel beef farmer who presented a submission on the changes, Ian Upston, said limiting nitrate levels on his farm would result in higher prices for food at the supermarket.
"The food producer is being targeted as being the polluter.
"Well, the answer is stop producing food.
"People will find out at the checkout counter at the supermarket with the law of supply and demand, less food being produced, up goes the price of food."
Mr Upston said it was unfair to apply the same stocking limits to his operation as a dairy farm as he only uses a fraction of the fertiliser.
Nobody from the dairy section of North Canterbury Federated Farmers was available for comment.