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The private boat (Free Spirit) hit a sandbank in Homestead Bay, at the mouth of the Matukituki River, about 3pm.
Wanaka Constable Aubrey Martin said five people were on board the boat, including the driver, on a scenic trip on the lake when the craft hit the sandbar.
With no other boats in the vicinity when St John arrived at the scene, a member of the ambulance crew walked through knee-high water to the boat.
St John spokesman Gerard Campbell said a 68-year-old male had moderate injuries, while two others had minor injuries.
A nearby River Journeys jet-boat picked up the injured people and transferred them to an ambulance at a point further up the river.