Christchurch water should be chlorine free again by the end of July, instead of the goal of May.
The council started adding chlorine to the water last May, due to concerns that some of the city's wells were at risk of contamination.
Although the council originally planned to have 90 percent of water in the city chlorine-free by the end of this May, water supply improvement programme manager Helen Beaumont said they faced a number of delays including finding that fewer wells were suitable for a quick temporary upgrade.
Ms Beaumont said to end chlorination in July the council needed to reconfigure its water supply network to mostly use pump stations supplied by upgraded wells, while keeping chlorination on standby until September in case of spikes in water demand.
"There could be times between July and September when we need to temporarily reintroduce chlorine to some parts of the city so we can meet demand and keep the water safe for residents while we complete our work," said Ms Beaumont.
A new ultraviolet treatment plant at the city's largest pump station, Main Pumps, was scheduled to be completed in June, but will not be finished until September due to the discovery of asbestos.
Ms Beaumont said the asbestos was not in contact with the water supply but did need to be carefully removed and disposed of before work could start at the site.
She said 75 percent of the city's water supply would be chlorine free by the end of May.
Chlorine treatment would continue to affect part of the central water supply zone until the end of July, including homes in Sydenham, Beckenham, Opawa, St Martins, Spreydon, Hoon Hay, Cashmere, Bowenvale and Huntsbury.