The boy, believed to be from Christchurch, was trapped inside and died.
Duncan said three adult males were airlifted back to Christchurch with minor injuries.
They underwent medical checks before speaking to police about the fatal incident.
Police said the Serious Crash Unit was investigating.
The boy's name has yet to be released.
The Ashburton Lakes area is about two hours inland from Christchurch.
Lake Clearwater, Lake Camp, Lake Denny, Lake Emily, Lake Emma, Lake Heron, Lake Roundabout, Lake Spider and two small lakes are situated in the region.