Waikato Regional Council says the region's water quality is deteriorating because of significant nutrient leaching and run-off from farms.
A report issued on Wednesday says contamination from various sources means almost 70% of the rivers and streams sampled may be unsafe to swim in. Water may be unsafe for stock to drink in 75% of the monitored waterways.
The report also says that the on-going use of phosphate fertilisers has led to a gradual build-up of cadmium and fluorine in some soils, which could restrict future land use.
Council chairman, former Waikato Federated Farmers president Peter Buckley, says that while farmers are already taking significant steps to limit their impact on the environment, there are still many serious issues that need to be addressed.
He says trends towards more intensive land use mean it's likely there will need to be significant changes in farming practice if the quality of Waikato waterways are to be protected.