NSW Health documents obtained by the ABC reveal areas where deadly pathogens are regularly detected at dangerous levels in unfiltered drinking water pumped from rivers, lakes and dams.
The water safety reports, obtained after a lengthy freedom-of-information battle, also show more than 100,000 NSW residents were issued protective boil-water alerts in the last five years.
Grafton, Kempsey, Scone, Jindabyne and Merimbula are cited as the five worst-performing areas, with repeated "contamination incidents" triggering "potential health risks".
Around Grafton, a population of 40,000 are at risk from cryptosporidium, a parasite that causes gastrointestinal illness.
Residents have faced 10 boil-water alerts since 2006, issued "in response to the inability of the water supply system to manage risks".
The documents say faecal contamination from cattle, and even swimmers along the lower Clarence River catchment, is the parasite's source.
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