A Kiwi is in hospital after an angry orang-utan bit him on the knee at a sanctuary in Indonesia.

New Zealander Binula Wickramarachchi, 27, was attacked by an agitated ape while trekking through the national park of Sumatra Orangutan Observation Centre.

The attack happened after one of the orang-utans swung down from a tree as Mr Wickramarachchi's tour group passed by, the Daily Mail reported.

Manager of the Sumatra Orangutan Observation Centre, Riswan Bangun, told the Jakarta Post that Mr Wickramarachchi was treated at the Colombia Hospital in Medan.

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"The victim was bitten on his right knee," he told The Jakarta Post on Tuesday.

It was believed a 38-year-old wild orang-utan named Minah was behind the attack, he said.

Minah was once kept as a pet by a resident, Mr Bangun told the Jakarta Post.

"She has been free for quite a long time and she has gone back to the wild in every respect," he said.