Radio-Canada says the diesel spilled into the sewers from a generator, and made its way into the river.
The problem appeared at about 4 a.m. ET, and the Ministry was notified just before 9:30 a.m. ET.
Quebec’s environmental emergency agency, Urgence-Environnement, is on the scene.
The Contrecoeur and Verchères municipalities, about 25 kilometres east of Montreal, take their drinking water from the river.
The ministry says town officials have been notified.
A statement from the Environment ministry says necessary measures are in place to ensure the safety of the public and to protect the environment.