People in Christchurch are being warned that extremely high levels of norovirus have been found in shellfish collected from river mouths around the city.
The contaminated shellfish were found in the Avon and Heathcote river mouths and around the estuary.
The Canterbury Medical Officer of Health, Dr Ramon Pink, says the discharge of sewerage into the two rivers as a result of the February earthquake has caused shellfish to become contaminated with norovirus.
Dr Pink says until further notice, people shouldn't collect or eat shellfish from anywhere in the Avon-Heathcote estuary or along the causeway.
Norovirus causes vomiting, diarrhoea and stomach aches as well as flu-like symptoms. The illness is usually brief, but can become life threatening for the very young, elderly or those with compromised immune systems.
Ramon Pink says so far outbreaks of norovirus in the community have been avoided due to good hygiene practices.