Toronto Fire Services were called to the scene at 17 Lascelles Boulevard shortly after 3 a.m. after reports of significant flooding at the address.
City crews have had to shut off multiple valves in an attempt to find which one would abate the flow of water into the building.
As of 6:30 a.m., the water was still flowing. The full extent of the damage is not yet clear.
Firefighters on scene told a CBC cameraman on scene that they were concerned the water could reach the building's electrical system and cut power to apartments.