People on Banks Peninsula's eastern bays should boil their water because it might be contaminated after heavy rain, Canterbury health officials say.
The water supply from Port Levy to Flea Bay could be polluted by run-off from farm land and septic tanks following last week's deluge, which could make people sick.
Most of the supplies in the area are private and are not treated or tested.
Canterbury medical officer of health Dr Ramon Pink said people heading to baches and holiday homes should not drink the water.
"If you live in the eastern bays of Banks Peninsula, or are visiting this area, do not drink the tap water - boil all water before using it for cooking, cleaning your teeth or for drinking. Or use bottled water," he said.
Pink said contaminants in the water could cause a gastrointestinal illness, with symptoms including vomiting, diarrhoea, headaches and a high fever.
Symptoms usually appear one to 10 days after people are infected.
Health officials are also warning people against swimming or collecting shellfish from Lake Ellesmere at Lakeside Domain, because of high levels of faecal bacteria.
"Water quality at affected site is not considered suitable for recreational uses including swimming because of the risk to health from the bacteria and other pathogens," Pink said.
"In most cases the ill health effects from exposure to contaminated water are minor and short lived. However, there is the potential for more serious health effects from exposure to faecal bacteria."
Water contaminated by human or animal faeces could contain a range of disease - causing micro-organisms such as viruses, bacteria and protozoa.
The water quality will be monitored weekly until it is safe for recreational use.
RNZ
