Hamilton City's water supply intake from the Waikato River has been turned off after 13 tonnes of potentially toxic fertiliser was spilled into the river.

A truck carrying 25 tonnes of urea rolled on Narrows Bridge on State Highway 21 and tipped around half its load into the river just before 8am.

A resource officer with Environment Waikato, Mason Jackson, says all downstream users of the river water have been advised to turn off their supplies as a precaution.

He says urea has the potential to turn into ammonia, and if this happens it can be poisonous to fish and other wildlife.

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