Water experts are advising the public not to panic, as arsenic exceeds acceptable levels in the Waikato River

Samples taken on Monday show the toxic chemical exceeded the allowable limit for the first time since at least 2002.

Authorities say drinking water is still safe to drink, and more tests will reveal the bigger picture this afternoon.

Water New Zealand's Belinda Cridge says arsenic has a risk of causing cancer, but only over long periods of exposure.

She says it isn't something to be concerned about in the short term.