The Canterbury Regional Council has just released research showing polluting nitrates from livestock could eventually make their way into the drinking water supply for large parts of Christchurch.
However the council said it would be at levels below the maximum safe standard set by health authorities.
Nitrates are harmless to adults but can be fatal to babies who can develop blue baby syndrome.
The Canterbury District Health Board currently advises new mothers in farming areas to not use tap water to mix baby formula because of high nitrate levels.
Its medical officer of health, Dr Alistair Humphrey, said while there had only been one confirmed death from blue baby syndrome, high nitrate levels were the canary in the coal mine for degrading water quality.
Today's report said there are currently minimal traces of nitrates in Christchurch's drinking water.
It uses models that predict it could show up at higher levels in 50 to 100 years from now but would remain below those harmful to humans.