Christchurch's water should be temporarily chlorinated, the district's top health officer says.
New engineering assessments of Christchurch's wells have revealed some of them may be susceptible to contamination, particularly in heavy rain events.
Christchurch's drinking water is drawn from 156 wells located at 56 sites across the city.
Latest assessments by engineers have raised concerns that some of the below ground wellheads may not be sufficiently sealed to prevent surface groundwater contamination, particularly in heavy rainfall events, Christchurch Mayor Lianne Dalziel said on Tuesday.
Christchurch lost its secure status for its water on December 22 2017, she said.
A programme of work is under way to get all wells upgraded by the end of year.
The Christchurch City Council head of 3 Waters and Waste John Mackie said, "There is no current issue with the quality of our ground water - it is safe to drink.
"The issue we face is how we can eliminate the slight risk of contamination occurring through our infrastructure."
Canterbury medical officer of health Dr Alistair Humphrey said the water was still safe to drink but there were "no barriers to contamination in Christchurch" and "temporary chlorination" should take place.
"We do not need to issue a boil water notice because the quality of the water remains good. The reason the council has to go through this programme of ensuring the wellheads are safe is because of the slight risk of contaminated water seeping into the groundwater."