Scientific testing of three rivers in Canterbury has revealed strains of a severe pathogen which can cause kidney failure and, possibly for the first time, antibiotic resistant E. coli.
The independent testing commissioned by Fish and Game has raised red flags for public health officials who say that it needs more investigation.
Samples were collected from above and below the biggest farms - all dairy - on the Ashley, Selwyn and Rangitata rivers in May and September and independently tested by Massey University's Institute of Agriculture and Environment.
Fish and Game commissioned the testing after anglers started questioning whether they could get infected from the river pollution and large numbers of dairy cows in Canterbury.
The findings showed the presence of an antibiotic-resistant E. coli and another dangerous strain of the bacteria called Shiga toxin-producing E. coli (STEC).
The two most common E. coli found in the testing come only from ruminants such as cows.
Fish and Game chief executive Martin Taylor said the first tests were so alarming it commissioned a second round to confirm the findings.
It it showed the contamination of Canterbury waterways was primarily due to intensive dairy farming, he said.
"The results indicate swimming in these rivers could be like playing Russian roulette with the health of you and your family," he said.
University of Otago professor of public health Michael Baker said STEC produced a toxin which could make you ill even if you swallowed only a few organisms.
The bacteria was almost absent from New Zealand 25 years ago, but the number of cases had risen rapidly in the last three years. In the last 12 months around 750 people had become sick around the country, and Dr Baker said it was particularly common in young children living in rural areas.
"This is one of the most severe forms of diarrhoeal disease you can get in New Zealand."
He said the bacteria was a serious worry for New Zealand.
Dr Baker said about a quarter of people who got infected were hospitalised and about 10 percent would get haemolytic uraemic syndrome which damaged kidneys, particularly in children.
"Some of these children infected will need renal dialysis and it can have long-term effects," he said.
He is recommending people to take precautions if they go swimming, such as not putting their heads under water.
Canterbury District Health Board clinical microbiologist Josh Freeman said the tests raised some red flags.
"We know that these bacteria can cause infections in humans and the fact that they appear, on the face of it, to be so common in these waterways certainly raises concerns that people may obviously be exposed to these organisms and become sick."
He said to his knowledge, it was the first time antibiotic resistant genes had been found in water in New Zealand and this needed to be investigated further because they could spread and make their way into hospitals.
Mr Taylor said the proper authorities urgently needed to undertake more testing to establish the extent of the problem, along with action to ensure people were kept safe.
Canterbury Regional Council said it was committed to working with other organisations, including Fish and Game, to protect and improve water quality.
It said its weekly recreational water quality monitoring, which included sites on the Ashley and Selwyn rivers, showed they were safe to swim.
Environment Minister, David Parker, told Morning Report since last year's Havelock North Water Inquiry, the government has worked on upgrading the testing of drinking water.
But he said dairy and beef cattle remain a significant part of the problem.
"In parts of New Zealand the level of intensity of farming in close proximity to waterways has gone too far and has caused these adverse effects.
"I was visiting a catchment where a group of farmers were coming together to try and help fix the problem, but I was told by the regional council that 90 percent of the farmers in the area were fully in compliance with their resource consents and plan rules, and despite that there were problems, which shows that tightening the rules is part of the answer."
Dr Parker said the ministry's Essential Freshwater Taskforce is working towards making a 'noticeable difference' in waterways.
"There'll be an updated national policy statement, plus a national environment standard which will have rules around some of the riskier practices.
"Part of the problems are some pretty aggressive farming practices in some parts of the country. Most farmers are up for improving these things, but there are some who are laggards and those laggards have to effectively be forced to comply with the law.
"[Just] changing the law doesn't fix the problem, you've actually got to have a change in land use practice on the land, which is causing the problem. On that I thought that the Fish and Game's (response) was of a different tone than it was a year ago, reflecting the fact that, a lot of the farm leaders, not all of the farm leaders, but a lot of the farm leaders have actually recognised that this problem has got too bad and they are up for the land use change."
Dr Parker said over time, he believes the type of land use near waterways will change.
"One of the ways we want to encourage that is to go to higher value land uses rather than cause the economy to tank, whilst meeting the environmental bottom line.
"In some parts of South Canterbury for example ... you will see, I think over time, movement back into mixed farming, including cropping, in some areas horticulture, which per hectare of land farmed are actually higher returning."