Arsenic occurs naturally in the Waikato River. Photo: RNZ/Leah Tebbutt
The Water Services Authority, Taumata Arowai, has confirmed slightly elevated - but safe - levels of arsenic in Waikato and Auckland's drinking supply.
The authority said the cause was not yet known, but tap water was still safe to drink.
Head of operations Steve Taylor said there could be many reasons for the unusual levels of arsenic in the river.
"We still don't know why the levels are higher but it could be naturally occurring or it could be a result of some discharge upstream into the river."
Slightly elevated levels of arsenic were also found in Waipā District Thursday, but the council said the water remains safe.
Taylor told Morning Report that geothermal activity could also be a potential cause.
"You typically find arsenic occurring naturally in areas of high geothermal activity around Taupō and Waikato river. There's a level [of arsenic] that comes out of the ground and is passed through rivers and streams naturally."
The Waikato council were investigating the matter and testing was under way to determine the location point of the contamination - which would help to determine the cause.
Taylor said it was important for people to understand what's in the water, if there were any changes to it but also how treatment was making a difference in keeping the water safe.
"It's important for people to understand where their water is relative to the [water safety] limits."
Watercare chief operations officer Mark Bourne said arsenic occurred naturally in the river and it was not unusual to see low levels in water samples.
"What is unusual is seeing it as this level - we've been sampling the Waikato river since 2002 and its the first time we've ever seen a result like this."
He said sampling would be done daily until the arsenic returned to normal levels.
Watercare's first response to the elevated levels of arsenic was to reduce the volume of water produced from the Waikato treatment plant, Bourne said.
"The volume that was reduced there was made up from other water supplies, so there's no need for Aucklanders to be concerned about either the quality, or the quantity, of water that's available."
National Public Health Service regional clinical director Dr William Rainger said drinking water with slightly elevated levels of arsenic for a short period of time was unlikely to affect people's health.
"People who are drinking water sourced from the Waikato River do not need to be alarmed. The limits in the New Zealand Drinking Water Standards are based on the level of risk from a lifetime of drinking the water."
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