A crew works outside one of White Rock's reservoirs, trying to find a source of E. coli contamination. (CBC)

Residents of White Rock, B.C., have been told they will have to keep on boiling their tap water right through the weekend, as officials continue efforts to decontaminate the city's water supply.

Last week low levels of E coli. bacteria were detected in the water supply, prompting officials to issue the boil water advisory on Friday night.

The advisory affects more than 20,000 residents in the city, about 25 kilometres south of Vancouver, and about about 80 homes in neighbouring South Surrey along 16th Avenue.

Epcor spokesperson Scott Lundy says crews have finished cleaning the Merklin Low reservoir, after three samples from it came back positive for E coli., and it is now being refilled before being tested again.

After that work is completed Epcor will still have two more reservoirs to decontaminate, and that will take some time, said Lundy.

"It just comes down to the amount of time that's required to essentially go reservoir to reservoir and clean and inspect and do all that work, and then get the water system back up and running," he said.

The company is also flushing and cleaning the rest of the water system's lines to ensure it is not contaminated.

"In addition to the cleaning and inspection, we've also done extensive water sampling and flushing of the distribution system, so that work has continued all the while we're working on our reservoirs," said Lundy.

Epcor still doesn't know the source of the e-coli contamination that was first found in water samples last week.

The Fraser Health Authority requires three days of clean water sample tests before it will consider lifting the advisory.