Health New Zealand has apologised and says it's a top priority to make sure a case of mistaken identity never happens again.

Police mistook an 11-year-old for a distressed woman in her 20s - and she was restrained and given drugs meant for adults at a Waikato mental health facility.

Chief clinical officer Richard Sullivan says it's clear human error was a factor - and they need to learn from this.

"What's clear here is - our staff were doing everything in good faith, working hard, there's no doubt some human error occurred here. But we just need to learn from that."

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