The water was cut off at 14 housing estates and other buildings in Tsuen Wan and Kwai Chung last night, with residents given just four hours' notice.
The Water Supplies Department said it expected service to resume after seven hours, at 6am. It said a supply system fault was to blame and repair work was being "urgently carried out".
District councillors said it would likely have minimal impact on residents, as the water was cut off overnight, but warned that there would be chaos if supply did not resume in the morning as scheduled.
Thirteen estates were affected in Kwai Chung, including all premises in the area bounded by Lei Muk Shue Estate, Shek Yam Estate, On Yam Estate, Shek Lei (I) Estate, Shek Lei (II) Estate, Yung Fung Court, Greenknoll Court, Kwai Shing East Estate, Kwai Shing West Estate, Kwai Luen Estate, Horizon Place, High Prosperity Terrace, Chun Kwai House, Ha Kwai House and Chau Kwai House of Kwai Chung Estate.
Buildings on Wo Yi Hop Road, Tai Pak Tin Street, Shek Yam Road, Tai Loong Street and Kwai Shing Circuit were also affected.
In Tsuen Wan, the water was cut off at Cheung Shan Estate, Sam Tung Uk Resite Village and Hoi Pa Resite Village.
The department would not say how many residents and businesses were affected.
On New Year's Day, the department suspended the freshwater supply in Western District on Hong Kong Island, causing disruption for residents and businesses. That suspension was due to a burst water main in Pok Fu Lam.
It said the main had been in use for over 30 years, and the incident was due to ageing. The department said it had been taking a "multi-pronged approach" to tackle the problem.