Waiheke police are guarding a badly decomposing body on Waiheke Island after it was discovered by a member of the public on Thursday.

It lies close to a bush walking track off Ocean View Rd, between the island's main village of Oneroa and the passenger ferry terminal at Matiatia.

Waiheke police acting sergeant Raymond Matthews said the body had lain undiscovered for some time.

"It's too badly decomposed to tell the sex or age of the person," he said.

He confirmed the body was being removed on Friday morning for post mortem in Auckland, a 35-minute ferry ride from the island.

He said the matter was out of the hands of Waiheke police, except for guarding the body overnight.

The case is being handled by Auckland CID sergeant Jacqueline Corner, he said.