Christchurch residents are being advised not to use part of the Avon-Heathcote estuary after testing showed the water is still contaminated following the earthquake on 4 September.
Sewage is still being discharged into the Avon River after damage to sewers in parts of the city.
Medical Officer of Health Alistair Humphrey says the water can contain elevated concentrations of human bacteria.
He says any contact with water in the northern part of the estuary is not recommended.
Dr Humphrey says sampling shows the water quality is improving four weeks after the quake.
A ban on water sports is in place north of a line from Sandy Point across to Plover Street.