Watercare says there's no public health risk, after high levels of arsenic have been detected in the Waikato River

The agency's been testing across the river, which supplies water to Hamilton area and some parts of Auckland.

Production's increased at the Ardmore water treatment plant, which takes water from the Hunua Ranges, while it is decreasing at the Waikato Water Treatment Plant.

Water Services Authority's Head of Operations Steve Taylor says it'd take very long-term exposure before this level of arsenic was a health issue.

He says it's safe to drink and it's expected these levels will reduce over time.

Results from testing are expected tomorrow.