Up to 40,000 residents on the NSW Central Coast were told to boil drinking water, after E. coli was previously detected in a reservoir, but new testing has cleared the supply of contamination.

Residents in some NSW Central Coast suburbs have been advised to boil their tap water due to the possible contamination of the local water supply with E. coli bacteria. A "boil water alert" was issued by Central Coast Council last night after recent samples detected the presence of E. coli bacteria.

An alert issued by Central Coast Council said samples from the drinking water supply contained E. coli bacteria.