Contamination has been found in the Richmond water treatment plant.
The cause of the contamination is yet to be identified but the Tasman District Council says the contaminants are in low levels.
A small amount of chlorine was added to the water last night in order to eliminate any risks to users. Residents may notice a slight smell and change in taste although council says the water is safe to use.
Chlorination will continue until early next week regardless of the outcome of further test results available today.
The Nelson Marlborough District Health Board has been involved in the decision-making.