The Wellington Regional Council says foul-tasting water issuing from taps in the city's northern suburbs would begin to improve on Saturday evening.
Residents of Wilton and Karori have besieged the Wellington City Council with complaints about the drinking water, saying it smells and tastes musty. Some of them have been buying bottled water instead.
Regional council water supply manager Chris Laidlow says recent heavy rain made Hutt River water unusable and water was taken from the reservoir lakes at Te Marua instead.
Mr Laidlow says engineers dose the water with carbon to get rid of the unpleasant taste and odour, but did not use enough this time. He says the algae is not harmful.
The council says as more residents try to conserve water, it is sitting in reservoirs longer, and engineers are looking at ways to ensure it turns over more quickly.