City officials say the break is near the intersection of Rothesay Avenue and Russell Street and has left many homes without water or low pressure.
The Department of Health has issued a boil water order for anyone living west of Rothesay Avenue and Russell Street.
The city says the order is expected to last until early next week.
It says two consecutive clean bacteriological samples collected a minimum of 24 hours apart are necessary before the boil water order can be lifted.
The city says to boil water for at least one minute before drinking.