High levels of E. coli bacteria have been found in parts of the Hutt River prompting a warning against swimming there.
Tests have shown contamination in the area of the river at Akatarawa Rd and Gemstone Dr in Upper Hutt is enough to cause diarrhoea, stomach cramps and vomiting.
The Medical Officer of Health, Margot McLean, says staff will search the area and run tests to identify the source of the bacteria.
The Greater Wellington Regional Council is also investigating the source of high bacteria levels in Rona Bay, in the Lower Hutt suburb of Eastbourne.
It is warning against swimming or collecting shellfish there until further notice.