The public health alert was issued on Saturday afternoon for people in New Brighton, Burwood, Wainoni, Aranui and Southshore after routine testing discovered total coliforms in the Rawhiti water zone.
But on Monday afternoon, the Christchurch City Council said it had three clear days of samples from the Rawhiti supply zone so most people no longer needed to boil their water.
More than 14,000 households in the city's east were under the boil water notice.
"Investigations are still ongoing but we believe it was due to work recently undertaken on the network," the council said.
"The council has done significant sampling throughout the zone and we are now confident the contamination is isolated to a small area."
The council will contact people who live at about 20 properties and need to keep boiling their water.
The total coliforms indicated potential contamination and the possible existence of harmful pathogens, but it was a group of bacteria that was commonly found in the environment, the council said.
'They've just been more safe than sorry'
Christchurch Mayor Phil Mauger said a little bit of dirt appeared to be the cause of the boil water notice.
Mauger said work had recently been done on a 50mm sub-main pipe connecting to the properties - about 10 metres away from a testing point.
"There's been a little bit of dirt go down this pipe. Now our testing gear is so good that it will test even half a thimble full of dirt and of course, it gives us this high result and we've got to boil notice on," he said.
"It's been flushed out but they've just been more safe than sorry on it."
The city had dozens of testing sites that were checked weekly.
"When we get a positive test comes up, we have no real option except to put in a boil water notice because we'd be in real strife if we didn't."
One testing site in the entire zone tested positive - the rest of the zone was fine but they had to be certain where the problem stemmed from, Mauger said.
"Now we know exactly where we are, we can really zero in on what this is even if we've got to dig the submain up and find out what the hell is going on. There's got to be a bit of dirt in there somewhere or something's happened that's caused this grief."
He apologised that inconvenience it had caused people.
No one has been reported sick following the contamination, Mauger said.
He did not rule out exploring a voluntary vulnerable persons register in the future, which was raised by the mother of an immunocompromised daughter who relied on sterile medical equipment for feeding.
