Drinking water in the Selwyn District is at increasing risk of contamination as more dairy cows are being farmed there, local residents have been told.

Photo: RNZ / Rebekah Parsons-King

About 300 residents turned up to a meeting last night, organised by a group campaigning for more action to look after Canterbury's waterways.

The meeting at Lincoln High School heard from speakers including Medical Officer of Health Alistair Humphrey.

He said water assessors employed by the district health board were doing their best to oppose irrigation schemes, such as Central Plains Water, because of the dramatic rise in the number of dairy cows they helped to facilitate.

But Dr Humphrey said they were encountering major resistance.

"A consistent complaint in the submissions from drinking water assessors has been that ... when they say 'we want to issue compliance here', they keep being told 'oh, we want a softly, softly approach because you mustn't upset the minister'."

Dr Humphrey said it was impossible for the regional council, Environment Canterbury, to meet its targets for reducing pollutants while at the same time meeting its other targets for increasing the amount of irrigation that was happening.

He said a soon-to-be released report from the council showed the level of polluting nitrates in ground water was increasing in 25 percent of wells.

"There's increases all the way across the Canterbury Plains, so we're not doing very well and we're not even hitting the targets we've set ourselves."

Dr Humphrey said high nitrate levels were potentially fatal for newborn babies, and this was something midwives were now factoring into their visits to expectant mothers.

"If those women have a private supply, as part of their booking they get those women to check their water supplies for E coli and for nitrates.

"They're able to check how well they're doing in their region and actually, sadly, for Canterbury it's generally bad."

Canterbury makes up 65 percent of the country's irrigated land, and Massey University freshwater ecologist Russell Death said the intensive dairy farming that irrigation facilitated was sending nitrate levels through the roof.

He said things would only get worse once schemes such as Central Plains Water were fully up and running.

"The nitrate levels in your waterways are far higher than almost anywhere in New Zealand, and it's almost getting to the point where they're so high that it's hard to know what you can do about it.

"It's in a really bad situation."

From next year, Canterbury farmers will be required to limit nitrogen loads on their paddocks, but even with the new rules, nitrogen loads are expected to increase by 50 percent within the next 20 years.