The Town of Saint-Bruno-de-Montarville, on Montreal's South Shore, is under a boil water advisory.
The first warning went out on Monday however the advisory has now been extended until at least Thursday.
Laboratory results indicate the presence of E. coli bacteria in a sample taken from the municipality's water system.
The bacteria can cause serious illness including vomiting, stomach pain, and diarrhea.
Municipal authorities are recommending all 27.000 residents boil water for at least one minute before use, or to use bottled water.
Unboiled tap water can be used to wash dishes with hot water as long as dishes are thoroughly dried afterward. It can also be used to wash clothes, and shower or bathe.
Splash Parks in the town are closed but pools will remain open.
Municipal authorities said a new notice will be issued when the water is once again safe to drink.