A Canterbury Medical Officer of Health says rivers are open sewers and people must take precautions to avoid getting ill.
The water supply to about 80% of the city were cut following Tuesday's 6.3-magnitude earthquake and sewage is going into waterways.
Dr Humphrey says there are no reports of disease outbreaks in the city at this stage, but people must avoid open water and be sure to keep their hands clean. He is also urging people to put on shoes to avoid cutting their feet.
He says Christchurch could avoid a disease outbreak if people are careful.
Dr Humphrey says the need to conserve water means sewage will have to be buried deeply or transported away.
Christchurch Bob Parker told a media conference neighbours may need to share water resources.
Mr Parkers says portaloos will be distributed but that is a "massive task" so in the mean time people will have to co-operate with neighbours.
Civil Defence has advised people to boil any water used, including rainwater.