Test results reveal drinking water in Waipā and Waikato District has slightly elevated arsenic levels - and Auckland's supply is back within safe levels.

Auckland's Watercare and Hamilton City Council yesterday revealed levels were fractionally above the maximum amount in recent samples from Waikato River.

Watercare's Chief Operations Manager Mark Bourne says they're tracking down the cause. 

"We've been able to share our results with the other agencies further up the river - and that'll help identify where, when and what might be causing this."

Watercare says tap water is still safe to drink.

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