A top doctor is urging pregnant women not to panic about high nitrate levels in Mid Canterbury water - despite earlier spotlighting the potentially fatal health risks.

Canterbury medical officer of health Alistair Humphrey has previously warned rural Ashburton residents of the serious dangers associated with some drinking water supplies, but said his comments were simply to raise awareness - not instil fear in the community.

Dr Humphrey said more action needed to be taken to protect the water quality in the district and pointed out high nitrate levels in some private and small water supplies was posing a risk to newborn babies.

In the worst situation, the high levels of nitrate could generate the fatal disease methemoglobinemia, otherwise known as blue-baby syndrome - affecting newborns up to three months - but only one suspected case had been reported in New Zealand.


"The important thing to emphasise is the town water supply is unaffected. Methemoglobinemia or blue baby syndrome is very rare, very rare indeed.

"However, what we are experiencing is a slow, increasing, but seemingly unstoppable rise in nitrate in some of our rural water supplies as farming intensifies over the years," Dr Humphrey said.

"We're looking at a juggernaut that is very difficult to turn around, but we need to stick together as a community and start making some serious changes to address this issue over many decades."

Ecan's team leader in groundwater quality Carl Hanson, who regularly samples the water quality, said the nitrate levels had been high for many years and there was no need for alarm.

Mr Hanson and his team tested the wells throughout the Canterbury Plains once a year, sometimes quarterly, and have annually recorded high nitrate levels in some shallow wells in the Ashburton area.

"We're talking about a very small portion of the community, but a very vulnerable portion too," he said.

"There should be no need to panic, this is not a new thing - nitrate levels in some water supplies have been high for a long, long time and we are continuing to sample them regularly."