Swimming may be ruled out in three Canterbury rivers following the discovery of a dangerous pathogen.
For the first time in New Zealand, a strain of antibiotic-resistant E. coli has also been discovered.
"New Zealand, you should be able to go out and take a swim in a river or go fishing in a river without the threat of having a life threatening illness," says Fish & Game chief executive Martin Taylor.
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Fish & Game commissioned independent testing of the Ashley, Selwyn and Rangitata rivers in May and September, following concerns about river pollution from dairy farming.
The findings reveal the presence of an antibiotic-resistant E. coli and a dangerous strain of shiga toxin E. coli carried by cows.
Dr Alistair Humphrey, Canterbury Medical Officer of Health, says the toxin can cause gastrointestinal problems, diarrhea and vomiting - and more serious issues.
"The problem with shiga toxin is that it can also cause kidney failure, requiring dialysis for the rest of the person's life."
All three rivers are surrounded by some of Canterbury's largest dairy farms.
"Individual dairy farmers haven't done anything wrong here," says Mr Taylor.
"What it is, as an industry, there are just too many cows. There's 1.3 million cows on the Canterbury Plain. You're going to have problems like this when you have so many cows, so the reality is we probably need to reduce cow numbers."
Environment Canterbury, who monitor the water, confirmed the findings match their own - but the levels of E. coli are well below their recreational use guidelines.
"What we have seen over the last four years is an improvement in the E. coli, and you can see that in the contact recreation gradings that are given," says chief scientist Dr Tim Davie.
"In the last four to five years we've seen an improvement, so that means less E. coli in the water - but it doesn't mean we're there yet. There's still a long way to go."
If you are keen to fish or take a dip in the rivers , there's some hygiene advice you should be careful to follow.
"The real message here is: if you are in these rivers in Canterbury - in any river in Canterbury - you have to be scrupulously clean about washing hands after you've been in the river," says Dr Humphrey.
"Anglers who have been out there with a packed lunch need to be particularly careful."