The waterline break occurred Thursday.
Starr said water samples tests were normal on Wednesday, but Friday's tests detected irregularities.
Additional water sampling was conducted late Friday and Saturday by health authorities, but no results were available by Saturday evening, Starr said. The water system will be tested again on Sunday, he said.
"I think it will be OK," added Starr. "In Plymouth's history of 57 years, we've never had E. coli."
Mason and City Council members went door to door Friday alerting the town of 400 to the boil order. The city also plans to leave letters on the doors of residents who are away for the Memorial Day weekend, Mason said.
A couple of people reported feeling stomach cramps, but Mason was not sure if that was from ingesting the bacteria.
The fire department is flushing the system and the city added chlorine to the water to help clean it, Mason said.
To disinfect the water, people should bring it to a rolling boil for five minutes. If the water has a flat taste after it cools, pouring it from one container to another several times aerating it will restore the flavor, according to the district.
The district encourages residents to call Plymouth city offices for more details on the boil order.