The San Bruno incident was reported shortly before 4:10 a.m. at the complex located at 701 Shelter Creek Lane, a water department dispatcher said.
The water main break is near Building 6 at the complex, according to the dispatcher, who did not have any information about possible damage caused in the incident.
Water department officials shut off water in the area and are investigating the incident, the dispatcher said.
The incident was the second water main break in San Mateo County this morning. The first was reported at about 1:50 a.m. in San Carlos.
That water main break occurred near the intersection of Rockridge Road and Sunset Drive and was contained by California Water Service Co. officials at about 3:20 a.m., according to San Carlos police Sgt. Eric Stanley.
The incident caused minor flooding in the backyard of one home, and did not cause any injuries, he said.