A water main break at Michigan's Great Lakes Water Authority's Lake Huron facility on Saturday has impacted an estimated 935,000 people, with 23 communities placed under precautionary boil water advisories. 

The GLWA said it had discovered a leak on a 120-inch water transmission main that distributes finished drinking water from the treatment facility to communities in southeastern Michigan. 

The main is the largest in the regional water distribution system.

One business in Greenwood and another in Imlay Township are also potentially affected. 

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A precautionary boil water advisories has been issued for the Village of Almont, City of Auburn Hills, Bruce Township, Burtchville Township, Chesterfield Township, Clinton Township, City of Imlay City, Lenox Township, Macomb Township, Mayfield Township, Village of New Haven, Orion Township, City of Pontiac, City of Rochester, City of Rochester Hills, City of Romeo, Shelby Township, City of Sterling Heights, City of Troy, City of Utica and Washington Township.

The community of Rochester Hills tweeted that, according to its Supervisory Control and Data Acquisition system, no loss of water pressure had occurred. 

"This means that you do not need to boil water at this time," it said. 

Flint, Michigan, which experienced a devastating water crisis beginning in 2014, was also impacted by the break – the cause of which is under investigation.

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The GLWA said crews had identified the location of the leak approximately a mile west of the facility and are working to isolate the area around it so that repair work can start. 

"Once the leak is isolated, crews will begin to open emergency connections to other mains in the system to restore some flow to the impacted communities," it said. 

The boil water advisory will remain in place until results from sampling verify the water is safe to drink.

Under the advisory residents should not drink the water without bringing it to boil for at least one minute and then letting it cool before use. 

Boiled, bottled or disinfected water should be used for drinking, making ice, brushing teeth and making food.

"Whenever a water system loses pressure for any significant length of time, precautionary measures are recommended since a loss of pressure can lead to bacterial contamination in the water system," the GLWA said. "Bacteria are generally not harmful and are common throughout our environment. Boiling water before using it will kill bacteria and other organisms that may be in the water."