The break occurred by the Roth Treatment Plant but impacted people around the city. Crews began working around 5 a.m. to get the leak isolated and the feeder line shut down.
Mayor Daniel Drew said about 1 million gallons was lost on the first hour, before the issue was isolated and the water was turned off.
During the day, those living in higher elevations, or nearby, had a higher likelihood of being impacted.
"We weren't 100% sure that the higher elevation areas were going to have water," said Drew. "We've ended up in a better position than we expected to be."
Middletown Water and Sewer Dept said they had a mix of problems: some customers lost water, others had low-water pressure and some were not affected at all.
As of early afternoon, water pressure throughout the city was almost back to normal. At least a half-dozen tanker trucks, from fire departments as far away as Stonington, where on standby throughout the morning in the Palmer field parking lot, in case of a major fire, but they were released just after noon time.
However, city health officials cautioned that business and residential customers could experience murky water for a couple of days.
"Flush your water using the cold water tap furthest from where the water comes into the house or to the apartment," said Sal Nesci, of the Middletown Health Department.
It has still not been determined what caused a pipe that was installed in 1965 to burst.
There has been very little business interruption in town, according to the Middlesex Chamber of Commerce.
Middlesex Hospital had to turn patients away from the ER and divert them to other hospitals. By noon, they were able to accept patients again, but all surgeries that were scheduled have been moved to Thursday.
The hospital still has two large water tanker trucks, from the Connecticut Water company, filled with 6,000 gallons of water each, parked on site just to make certain that they have plenty of drinkable water for use in the kitchen and food prep area.
Residents are being asked to conserve water. Repairs could take most of Wednesday to complete. A pipe has been brought in to replace the broken one.
The leak forced Middletown Public Schools, Middlesex Community College and Vinal Technical High School to close Wednesday.
The Middlesex YMCA was closed but reopened at 1 p.m.
The John S. Roth Water Filtration Plant was constructed in 1967. There are 10 active wells in that area that serve about 80 percent of the city.