Groundwater from private bores may contain higher than normal levels of naturally-occurring arsenic, the Hawke's Bay Regional Council said in a statement.
The affected groundwater, recently confirmed to be found at three Hawke's Bay locations, fails the national drinking-water standard of 0.01 milligrams per litre of potable water.
"Long term consumption of water with arsenic concentrations greater than 0.01 mg/L can cause adverse health effects, even increasing the risk of cancer," says Principal Groundwater Scientist Janine Barber.
"Unless specifically testing for levels of arsenic, anyone who has recently tested their self-supply water quality will not know whether high levels of arsenic are present."
The elevated arsenic levels occur naturally in groundwater due to the presence of specific materials in the aquifer, and is local only to specific bores.
The Regional Council has advised self-supply bore owners to test their water for elevated arsenic levels.
Members of the public seeking more information on potential health risks related to elevated arsenic levels can contact the on-call Health Protection Officer on 06 878 8109.