The Prime Minister is promising constant, and transparent communication - over arsenic levels in water from Waikato River

Slightly elevated readings of the toxic heavy metal have been detected in treated water from the river.

More tests are due back today.

Chris Luxon says arsenic occurs naturally in bodies of water, but officials are keeping a close watch in case the levels continue to elevate.

He says people will be alarmed -- but it's just a slight elevation off very small levels - and there is no risk to public health, and the water is safe to drink.