Customers in North Valley Stream, Elmont, Floral Park, Franklin Square, New Hyde Park, Stewart Manor and Garden City are being told to boil their water before drinking it.
Separate water samples tested positive for E. Coli and Total Coliform bacteria, prompting the warning.
Notices are being distributed door to door Wednesday night and Thursday morning.
Testing done by the Nassau County Health Department since the positive samples have shown no traces of either bacteria.
Customers should boil water for one minute and then allow it to cool before drinking, making ice, brushing teeth, washing dishes or preparing food.
The Water Authority will be flushing the water mains in the area of the contamination and the warning will be lifted when their tests show no bacteria.
For more information, please contact Customer Service at 516-327-4100 or 58 S. Tyson Avenue, Floral Park, NY 11001.
If you require further assistance call the Nassau County Department of Health at 516-227-9692 or after hours number at516-742-6154.
General guidelines on ways to lessen the risk of infection by microbes are available from the EPA Safe Drinking Water Hotline at 1(800) 426-4791.