Residents of St. John's who spent a frustrating day Wednesday without water have the service back but are being urged to boil their drinking water for at least 24 hours from the time their taps started running again.

Water was cut to thousands of households and businesses in the east and central areas of the city just before 7 a.m.

City crews scrambled to find what they suspected was a major water main break, with workers driving up and down streets looking for signs of water flowing from the ground. Within two hours, the problem was tracked to a main water pumping station at Windsor Lake.

Paul Mackey, the director of public works for St. John's, said water was slowly returned to residents in the central and east ends of the city around noon Wednesday after a technical problem was fixed at the main water pump building. (CBC)

"There was some technical glitch out there which caused an interruption in the flow, in the supply, to the system," the city's director of public works, Paul Mackey, told CBC News shortly after the source of the problem was identified.

An investigation revealed a more detailed explanation of what happened, said Art Cheeseman, the city's director of engineering.

Normally, as the water from Windsor Lake is filtered through the treatment plant, sensors monitor the system to make sure everything is working properly before the water is sent to homes, Cheeseman said.

It appears that at one point Wednesday morning, more air than normal was detected in the filtering chambers, and as a result, the system shut itself down.

"It appears that a level sensor actually tripped out the plant," Cheeseman said.

He said the sensor has now been replaced, and workers will monitor the system throughout the evening to make sure the problem has been fixed.

Water started flowing to homes by noon, with the pressure slowly returning to normal and water fully restored to all neighbourhoods by midnight Wednesday.

The boil-water advisory has been put in place for areas east of Columbus Drive, from Bay Bulls Road to the Prince Philip Parkway, including the Thorburn Road and Allandale Road areas.

"People who've had their water restored are probably noticing that there is discoloured water, and that means, where we're recharging the mains, any sediment in the mains is being stirred up," Cheeseman said. "So, we're asking people to run their water, and as a precautionary measure only, we're suggesting that people boil the water for the next 24 hours."

He said water should be brought to a boil for a couple of minutes before being consumed.

The lack of water led to nearly two dozen schools and many businesses being shut down because of the water problems.