Routine water testing today has picked up the presence of E.coli at Lower Hutt's Naenae reservoir.
Wellington Water advises that residents may wish to boil their water as a precaution, but it’s not a requirement.
The agency says standard procedures in such cases include isolating the reservoir and/or increasing the level of chlorination, as well as carrying out further testing of the affected area of the network.
As such, Lower Hutt residents may notice an increase in the taste and odour of chlorine in their drinking water over the next couple of days as chlorination is increased.
Anyone experiencing nausea, vomiting or diarrhoea should seek medical advice from their GP or after hours medical centre, or call the Ministry of Health Healthline at 0800 611 116.